Firefox has some of the best addons out there. Some of my favourites include DownThemAll, NoScript, Flashblock, Video DownloadHelper, and Greasemonkey, to name a few.
Firefox addons are more powerful than their Google Chrome counterparts. However, I woke up yesterday to I find out that Firefox is deprecating XPCOM and XUL-based addons.
What does this mean? A lot of addons won't work anymore. As mentioned on this post, addons such as DownThemAll will likely not work and the developers will probably not support them anymore. This includes any addons that let you do more "advanced" things.
|Firefox is turning more and more into Chrome|
The only upside to this is it will be easier for developers to make addons that work in both Google Chrome and Opera. However, if I wanted to use Chrome or Opera addons I would use Chrome or Opera, not Firefox!
Firefox is the most customizable and feature-rich web browser, and it should stay that way.
Mozilla, listen to your users.