All-in-One Sidebar – Puts all of Firefox’s separate windows (Add-ons, Downloads, History) in the Sidebar. Toggle with F4.

Auto Copy – Select text in Firefox and its automatically copied to the Clipboard. So useful, that I do it in other applications and forget to press Control-C.

Download Statusbar – Creates a new status bar at the bottom of Firefox that appears when you download a file. No more Downloads Window (or Sidebar). Double click to open the file.

Flat Bookmark Editing – In the Bookmarks Manager, it enables easy editing of Bookmark information.

Mouse Gestures – Control Firefox by rightclick-dragging your mouse in certain directions. I pretty much only use the rightclick-drag to the left for Back.

PDF Download – Short circuits PDF links and asks you if you want to download it to your computer, view it in the browser or open with Adobe Reader.

Tab History – Click a link that opens a new tab, and that tab will keep the history of the parent tab!

Tab Mix Plus – Tons of Tabbed Browsing preferences, defaults are cool, but you can download my settings file and import it (in this extension’s Settings) to get a few more tweaks. This extension’s Session Saver is better than Firefox’s default.

Table Tools – Sorting of tables in any webpage by Control-Click the header, right click the table to copy it as tab-delimited text for easy import into Excel.

For further (and possibly geeky) Firefox tweaking, check out these 15 cool tips. Ask any questions in the comments and I’ll be sure to try to help you out. :)