I’ve been using Firefox exclusively for about a year now. Here are some of the plugins that I find useful:
Fire Uploader – Allows you to upload media to your YouTube, Flickr, Facebook accounts from a single page.
TabMix Plus – Enhances Firefox tabs, including saving and restoring tab sessions.
Flashblock – Stops flash objects from playing until you click on them.
IE Tab – Opens a tab using Internet Explorer’s page renderer. Useful for those annoying sites that only work properly in IE.
Rikaichan – Translation tool for Japanese-English. It also does Japanese-Russian, Japanese-French, and Japanese-German.
Google Toolbar – Same as the toolbar for IE.
Customize Google – Forces GMail to use a secure connection, removes ads, and various other Google tweaks.