Do you use Firefox 4 already? If so, you probably know that a lot of addons don't work with Firefox 4. And it's not like it was with older Firefox releases: just forcing the addon to install won't make it work as the code in Firefox has changed and there's nothing you can do to make them work.

WebUpd8 reader Justin created an Firefox 4 compatible add-ons collection. The addons in his collection also play nice with Linux (Ubuntu specifically). Check out the collection @ addons.mozilla.org.

Further more, Stylish and Greasemonkey as well as some other extensions from addons.mozilla.org do not work with Firefox 4 beta 4. However, Justin searched for builds which work on the devs websites so you can get those to work too. Check out his blog post for further details.

Mozilla released an experimental Firefox addon called "Contacts" which allows you to import Twitter and Gmail contacts (for now) and the Mac OS X address book into Firefox. This is a first step for globalization (contacts-wise):

Why is this information important? Address books and buddy lists have become an integral part of how we manage our relationships online. Your email addresses, Twitter handle and openID are used in hundreds of ways as you connect, follow and share online.

Every desktop operating system, and hundreds of web-based service providers, has some way of representing “who-you-know”. Managing all of this information can be a lot of work. There are multiple copies of your data, and they are inconsistent and disjointed.

Mozilla has finally released Weave 1.0, the Firefox extension that provides Firefox user data synchronization which includes complete syncing of bookmarks, passwords, preferences, history, and even open tabs on multiple computers (or multiple operating systems) and even mobile devices.

Mozilla Weave Sync lets you synchronize your browser (including Fennec) across multiple computers. Also, it is likely that Firefox 3.6 or 3.7 will integrate Weave Sync by default.

Mozilla Weave Sync 1.0 release candidate is now available for download, and among the improvements are:

Install Mozilla Weave Sync

Some time ago, we talked about CookiePie, a Firefox add-on which allows you to use a new session so that you can log in to the same service with multiple accounts (such as multiple Google accounts, etc). But CookiePie doesn't always work and it breaks the cookies if you move the tabs around.

Multifox is another Firefox extension for logging in to multiple accounts on the same site which I like because it doesn't messes with your cookies and I was never logged out from both accounts like it sometimes happens with CookiePie.

Automatically download certain file types to the folders you set. For instance, you can set to automatically download all the .png and .jpg files to your Wallpapers folder, all the .zip files to the Downloads folder and so on. The filters are based on filenames and hosted domain's name.

With this Firefox add-on, you can capture the Firefox window or the whole screen and make screencast videos (it also records your voice). Using it, you can compose tutorials, e-lessons or videos. Unfortunately, this extension does not work on Linux.

Gain some space by hiding the bookmark toolbar until you move your mouse over the area where the toolbar was.

Provides a progress meter for each tab - Firefox 3.7 style. Unfortunately, this extension is incompatible with TabMix Plus.

Pin favicon-sized app tabs to the left side of the tab bar. Inspired by a feature by the same name planned for Firefox 4.0! With this extension installed, just Ctrl+Click the tab, select App Tab, and any tab will shrink down to an icon only on the left of your Firefox window. You can set it to retain your App Tabs between sessions and protect them from closing.