Most users that favor Firefox do so because of the myriad of plugins that are available. Below are the plugins that I currently use.
Classic Compact: Technically a Firefox theme, Classic Compact tightens up the Firefox user interface to allow for more website real estate. Every pixel counts!
Delicious Bookmarks: After searching and surfing the web for years, I’ve some across hundreds of sites that I want to save for later use, maybe a year or more later. The social networking and bookmarking service Delicious allows you to save these and tag them to be easier to find. The Firefox plugin adds buttons to the toolbar to make it even easier to tag sites. Much better than using favorites or bookmarks.
Foxy SEO Tool: If you are a web professional, or are doing in depth site analysis for you company site, it does not get much better than this. Traffic, profile, keyword, and other tools are all available. Probably the best all-in-one SEO package for Firefox
RunDownload: Like to “run” or “open” a download like in Internet Explorer? This plugin adds that capability to Firefox. Since this is an “experimental” addon, be sure to check the box to allow it to be added to your Firefox installation.
SearchStatus: Another SEO tool. Quickly see the PageRank of a website upon visit.
Tab Mix Plus: Unbelievable control of Firefox tabs. Almost overwhelming with features.
Xmarks: Have 2 or more computers that you want to keep bookmarks and passwords synced between? This is as good as it gets.