Add-ons:
Adblock Plus
NoScript
Flashblock
Lazarus: Form Recovery
Rehost Image
Video DownloadHelper
IE Tab 2 (FF 3.6+)
#2
Dave
You know, I don't really have any themes. I should look into that.
#3
AshburnerX
I love Echofon, as it lets me tweet without needing to check my twitter feed every 5 minutes.
#4
figmentPez
I've been using Qute as my Firefox theme since it was the default back when Firefox was Phoenix.
#5
DarkAudit
No themes, but Adblock Plus, NoScript, Torbutton, and FoxyProxy are must haves if you want protection and access to content (I'm looking at YOU, BBC iPlayer and Hulu!)
#6
Chibibar
I use two.
Adblock plus and no script no theme.
#7
Bones
laughing man theme(from GiTS), anything and everything to shoot down banners, flash crap, and pop-ups
#8
Calleja
I can't live without Ubiquity, Adblock, Tab Mix Plus, Greasemonkey and Stylish... I have several others but they're not as indispensable.
I'm really happy with Kempelton as my theme... BUT I also have an extension called GlasserPojo which makes the frame around Firefox look Aero-ish, so it looks AWESOME in Windows 7.
Oh, yeah, I also have everything but the navigation and tab bar hidden.. I like my screen real estate.
#9
Hyimi
For home, I LOVE adblock plus, site launcher, greasemonkey and greasefire, just to name a few.
For work (I am in IT) I absolutely love my Nagios Checker. It parses data from my Nagios network monitoring software and puts it into a small space in my statusbar. Basically it's green for no problem, yellow for a non critial problem, and red for critical. I can filter out things I don't need like machines that are turned off every night (otherwise shows up critical) and change how info is displayed. I click it and it takes me straight to the alert page on my Nagios web interface. Very nice for alerting me to network and system issues before I even get a call about it. Most of the other add ons are for little things, nothing major, like when viewing the page source for a website, it opens in a new tab rather then a new window.