This is a list of all the Mozilla Firefox plugins or so called extensions I use:

• AdBlock - Very extensive adblocker (not only blocks, but also removes ads from html!) (install)
• CompactMenu - Remove the menu and turn it into a little shark (install)
• SearchStatus - Add Google Pagerank and Alexa ranking to the statusbar (install)
• UserAgentSwitcher - Change the identity of FireFox to resembe IE or anything else (install)
• WebDeveloper - Very extensive web development bar, adds alot of nice features (install)
• GoogleBar - Adds a toolbar from which you can search Google, which rocks (install)
• FLST - Short for Focus Last Selected Tab (and some other nifty features) (install)

Next to this I also use a few plugins which are available in the Debian GNU/Linux distribution i'm using:

• flashplugin-nonfree - The MacroMedia flash plugin for Linux.
• sun-j2re1.4 - The Java 1.4 Runtime Environment for Linux
• mozplugger - Enables all sorts of inline apps like movie-viewer with mplayer and PDF viewer in Mozilla

For the Java part, click on the link, there's an explanation on how you can install it on Debian systems. The rest is just apt-get install'able! Which is why i love Debian :P.