Firefox collections are groups of related add-ons assembled for easier sharing.  This idea was launched from the Firefox Add-ons site several months ago.  Since some time has passed, I thought it might be interesting to check back in with this online Mozilla feature to see which collections are the most popular out there.
Here are the most popular add-on collections for the Firefox browser:

Windows 7 Theme for Firefox 3.5
32430 subscribers – This collection of add-ons were gathered to [...]

Need to filter your Facebook information? 
Have to many friends, or do you just not care that Bobby ranked up in Mafia Wars or Julie found a cat in FarmVille? Thanks to the Facebook Filter add-on for Firefox, you can customize the kind of content you see from your Facebook friends.

Do you use a lot of Google services?  The Firefox add-on, GButts, will help you get quick and easy access to any Google service you want, next to your address bar or anywhere else you might need it.
Here are all the Google services you can quickly show or hide via the extension.

You can customize how much space you want to save with the options across the bottom (by making the menu a toolbar or a drop down menu).  You [...]

You can drag your tabs around and organize them like you want to have them by default. There is no doubt this is one of the more useful features Firefox has put in the browser in some time. What about grouping your tabs by the source? You can’t do that until you pick up the Separate Tabs extension for Firefox.
+ Download and Install the Separate Tabs Extension for Firefox!