Mozilla has just passed a massive milestone, with its two billionth Firefox add-on being downloaded from its Add-ons for Firefox website.

“When we reached 1 billion Firefox Add-on downloads in November 2008, we thought it would take us less than 3 years to get to the next billion, and with your help, we’ve gotten to 2 billion in half that time! With more than 150 million add-ons in use every day, we know that the next billion add-on downloads will be here before we know it” wrote Julie Shin Choi, on behalf of the Firefox Add-ons Team.

Mozilla’s implementation of add-ons for Firefox is by far the best solution, giving add-on developers much more control and freedom compared to add-ons and extensions available for Chrome and Opera. This implementation is the key to why Firefox has so many users today.

Any Firefox users who are not using add-ons are really missing out, with add-ons available for just about everything you can think of, from download management, to social and communication, and even to web development.

Firefox add-ons are free, and are checked by Mozilla before they are released to the public. You can start browsing for add-ons on the Add-ons for Firefox website.

Mozilla has renamed its Weave add-on to Firefox Sync. The plugin allows users to synchronise their password, bookmarks, history and tabs across multiple devices.

“We are excited to announce that the “Weave Sync” project from Mozilla Labs has officially graduated and is now being incorporated into the Firefox roadmap. “Weave Sync” is now named “Firefox Sync” and the service will become a feature of Firefox in an upcoming major release” announced Mozilla’s Ragavan Srinivasan.

The add-on for Firefox has already had over 1.4 million downloads, and is proving to be hugely popular. A similar feature is available in both Google Chrome and Opera.

Currently available in more than 15 languages, the Firefox Sync add-on can be downloaded from the Mozilla Add-ons website.

Rumors are starting to circulate suggesting that extensions might be on their way for Apple’s Safari browser, bringing it into line with other browsers such as Firefox and Chrome.

Technology enthusiast and blogger John Gruber has alluded that such an announcement might be made at this years Worldwide Developer Conference (WWDC).

“The other big thing that’s missing (compared to both Chrome and Firefox) is a proper extension API. If only Apple had an imminent developer conference where they could unveil such a thing” wrote Gruber.

While not exactly confirmation, the possibility of an announcement at this years WWDC is entirely plausible, and would be a very welcomed addition to the Safari browser.

We will keep you updated if this story continues to develop over the coming weeks.

Along with the customisable tab bar, Firefox 4 also looks set to gain a new add-on manager, which is currently receiving a major overhaul.

The new add-on manager works in a browser tab, removing the need for a separate window completely. Along with the standard options for managing extensions, themes, and plugins is the ability to manage both installed languages and search engines.

Currently it is still very much a work in progress, with not all functions working as intended, with  icons and the interface still not final. Other features are still yet to be included, such as silent extension updates.

The new add-on manager is expected to start appearing in nightly builds of Firefox 4 soon.

Mozilla appears to have let it’s guard down, with a Firefox add-on that included a Trojan that could allow remote access to a users PC.

Two add-ons were affected; Master Filer which was infected with a password-stealing Trojan called Win32.LdPinch.gen, and Sothink Web Video Downloader which was infected with a backdoor Trojan called Win32.Bifrose.32.Bifrose.

Mozilla has issued a statement:

“If a user installs one of these infected add-ons, the trojan would be executed when Firefox starts and the host computer would be infected by the trojan. Uninstalling these add-ons does not remove the trojan from a user’s system. Users with either of these add-ons should uninstall them immediately. Since uninstalling these extensions does not remove the trojan from a user’s system, an antivirus program should be used to scan and remove any infections.”

Mozilla believe only 4,600 people are infected after downloading these add-ons.

How these add-ons made it online is unknown, as Mozilla scans all add-ons for viruses before they are approved. Mozilla now plans on using two different malware detection tools to try and stop this issue from reoccurring in the future.

With Chrome extensions having been available for one week now, we thought it would be good to take a look at the 10 most popular extensions for the week.

Collectively, these top 10 extensions have almost had just under 500,000 million downloads.

Google Mail Checker

With over 108,000 users, this extension displays the number of unread messages in your Google Mail inbox. Clicking on the icon will open your inbox. Currently only gmail and googlemail domains work, so anyone using a personal domain with Google Apps is out. There is also a reported memory leak at this stage with some users suggesting the less popular (but just as good, if not better) Google Mail Checker Plus.

AdThwart

AdThwart has almost 70,000 users, with the extension blocking ads from websites, thus saving you bandwidth and valuable screen space. The ad filter list is constantly updated by the EasyList community.

Google Translate

With 63,000 users, Google Translate does exactly that – translates. Web pages can be translated into your chosen language with a single click. This extensions users Google Translate.

IE Tab

IE Tab has over 53,000 users and is of great use for web pages that do not load or open correctly in Google Chrome. As the name suggests, this extensions lets you open an Internet Explorer tab right in Google Chrome, without the need for changing browser.

Google Wave Notifier

This extension has almost 50,000 users, and is similar to the Google Mail Checker. With Google Wave Notifier you can see all your unread Waves, as well as accessing specific Waves with a single click. It’s also compatible with Waves setup on a Google Apps domain.

RSS Subscription Extension

RSS Subscription Extension has over 46,000 users, and allows users to subscribe to RSS feeds with a single click. 4 feed readers are defined by default (Google Reader, iGoogle, Bloglines and My Yahoo), but others can be added manually.

Xmark Bookmark Sync

With just under 40,000 users, this extension allows you to backup and sync bookmarks across computers and browsers. The extension is also available for Firefox, Safari, and IE.

Docs PDF/PowerPoint Viewer

This extension has over 37,000 users and allows users to automatically preview both PDF and PowerPoint documents in Google Docs Viewer. This can save you lots of time, especially if the document is rather large.

AdBlock

Famous among Firefox users, AdBlock does what it says – it blocks ads from websites. The extension has over 36,000 users and the block list is automatically updated. This is the second ad blocker in the top ten.

Google Quick Scroll

Rounding out the top 10 is Google Quick Scroll, with almost 33,000 users. Google Quick Scroll lets users jump directly to the important bits when performing Google searches. After clicking on a search result, and when deemed to be useful, a small pop-up window appears showing one or more bits of the page that are relevant to your search. An obvious time saver!

That’s the top 10 extensions one week on, with extensions being available for Chrome Beta on Windows and Linux. Mac users unfortunately miss out at this point. Many, many more extensions can also be found on the Google Chrome Extensions website.

Google Chrome Gmail Checker Extension

With the latest Beta releases of Google Chrome today, Google have also released Extensions Beta.

Available to Beta Chrome users on Windows and Linux (with Mac still in progress), over 300 extensions can be downloaded at today’s launch.

Extensions can let users check their Gmail, translate web pages into different languages, and watch eBay items all without the need to visit actual websites.

Extensions have flourished in Mozilla Firefox, otherwise known as add-ons, and have helped grow the browsers fan base.

The extensions are all free and can be downloaded from the Google Chrome Extensions website.

Mozilla Labs have released a new project called Jetpack. The project is a  “newly formed experiment in using open Web technologies to enhance the browser, with the goal of allowing anyone who can build a Web site to participate in making the Web a better place to work, communicate and play” says Mozilla.

The aim is to allow web developers to design add-ons for Firefox using just HTML, JavaScript and CSS.

A download for Firefox is available along with more information, documentation, and videos from the Jetpack website.

Mozilla has today released Fennec Alpha 1 for Windows Mobile. Until now, to use the new mobile browser you needed either a Nokia N800/N810 or an HTC Touch Pro.

This new release worked with Windows Mobile 6 devices and includes new memory management technology (the same used in Firefox), a user interface designed with CSS, and support for add-ons.

A full list of known bugs, changes since pre-alpha builds, and installation instructions can be found in the release notes.

With the latest developer build of Google Chrome being released this week (version 2.0.180.0), evidence has surfaced showing that extensions and plug-ins are on their way for the browser.

Release notes show that developers have implemented “extensions window & tabs API events.”

“The extensions posse would like to point out that as of today’s dev channel release, extensions are starting to be a bit more useful. We can now put little bits of UI (user interface) in the chrome of Chrome, and some of the APIs (application programming interfaces) are starting to come together,” said Google programmer Aaron Boodman. “There is still quite a ways to go, but if you’re interested in building extensions for Chrome, this might be a good time to start taking a look.”

Google has also updated Chrome Extension HOWTO and included some sample extensions, including a gmail checker.

You can download the latest developer release from the Chromium website.