I was running Firefox 3.6 with the following:

BarTab
Firefox Sync (Now integrated into Firefox 4; no longer needed as a
separate addon)
GreaseMonkey
HTTPS Everywhere (EFF's addon based on NoScript design that forces
HTTPS for listed sites; easy to add your own rules)
NoScript
TabKit (tabs on the left side, grouped based on how they're opened,
indented as well; and color-coded by group)

Did some testing with Firefox 4 (on one machine), discovered a few problems.
BarTab half-works (it will still NOT load un-targeted tabs at launch
time; but you have to manually reload any 'unloaded' page to get it to
work; no other noticable issues - I don't auto-unload stuff after
timer though)
TabKit's Developer has been AWOL since October; so I had to move over
to Tree Style Tabs - which emulates TabKit with the exception of the
colored groups.

I run Chrome for a few things (namely a few Google-centric things) but
avoid running it for two reasons: They DO track the history of
EVERYTHING in there - and their now-non-support for the "Do Not Track"
feature that Google is now (AFAIK) the sole browser maker to say
they're not implementing it.  (Firefox 4 and IE9 have it, Apple's
announced Safari will in the next major release; don't know yet about
Opera though.)
https://secure.wikimedia.org/wikipedia/en/wiki/Do_not_track_header
http://donottrack.us/

That, and I've used Firefox for a long time - it'd be a massive pain to switch.

I'm still using the official Firefox Sync servers for now; I
eventually plan to set my own up at some point.

Another useful thing is the 'Addon Collections' you can create.
(Requires an account on the addons.mozilla.org site)  You can
pre-build a set of addons to install at once as a bundle.  Be it for
your own use, or just a pre-set list for users you support.  I'll be
creating two, one for me and a 'lite' version for family and friends
(They would get frustrated at NoScript, for example)

Keith Bachman