Firefox.

It turned out I couldn't live without Ubiquity. Still under development, although officially it's on hiatus.

The Add-On Compatibility Reporter, so I can run edge versions of extensions.

Firemacs (for sensible Emacs bindings)

TabKit (tabs on left side and tree style, with grouping and so on) and Ctrl-Tab (LRU tab switching, plus thumbnailed tab display and switch to tab by name-typing). All other tab organization systems pale in comparison.

And the rest: NoScript * Firebug * Web Developer Toolbar * Greasemonkey * IETab * Vacuum Places DB