These are the ones i'm currently using:

[FlashGot] can handle single and massive ("all" and "selection") downloads with several external Download Managers.

[Tab X] adds one button for each open tab.

[Extended Statusbar] is just that... :)

[Image Zoom] adds zoom functionality for images.

[SubmitToTab] adds an option to your context menu to submit an online form to a new tab or window. Quite useful.

[Menu Editor] allows you to customize all the menus in Firefox.

[IE Tab] embeds M$ Internet Exploter engine inside Firefox.

[Fasterfox] allows you to make some industrial strength performance adjustments.

[FoxyTunes] adds audio player controls to Firefox. I'm also using [OnyxOrbs] skin for this one.

[Nightly Tester Tools] for installing "incompatible" extensions mainly.

[LinkVisitor] simply to mark a link as visited or unvisited.

[OpenBook] allows you to customize the Add Bookmark dialog.

[Adblock Plus] to kill banners/flash junk.

[PDF Download] allows you to choose either to save the PDF file directly, open it embedded in the browser or to render it as HTML.

[Video Downloader] allows you to download videos hosted in Youtube, Google Video and such.

[Update Notifier] notifies you when updates are availabe for your extensions and themes.

[Colorful Tabs] is my latest addition. This one colors every tab in a different color and makes them easy to distinguish.

Labels: Software