I've been firefox (or its predecessors firebrid etc) for quite some time now. The latest installment of firefox, version 1.0 caused me a couple of headaches as a whole bunch of my extensions stopped working. As a result, I removed all of my personal settings and started anew with my firefox install.
Here are some of the settings I changed
Use a blank page as home page
Change all serif fonts to sans-serif. I don't like curls and all in my fonts when I'm browsing.
Change the download folder to ~/downloads
I don't want to see any download manager as they clog my desktop
Tabbed browsing is the bomb. I'm an avid user of ctrl-T, ctrl-W, ctrl-pageup, ctrl-pagedown, ctrl-shift-pageup, ctrl-shift-pagedown, and if you don't know of these shortcuts in firefox, try them out, now!
Important settings that I like:
Single window mode, i.e. never allow firefox to ever open a second browsing window, I'm a tab fanatic (I don't want to get lost in 10 browser windows, I have a regular place for the places I usually visit)
For a new tab, open a blank page and put it just right of the tab I'm currently using (no use putting it in the end, this option gives me better control when initializing new tabs)
If the current tab is closed, bring me the tab I selected last. No use going to a random tab in my list of tabs
When I close a window, just close all tabs. I only have one window after all, so I can't be bothered with the tabs when I want to shut down firefox
Show a closebox in any tab
I'm very fond of the tab groups mode, where tabs are arranged into groups using a coloring scheme in the tabs
I'm very much relying on google while I'm working. I consequently want a good arsenal of assault weapons when I'm undertaking a search mission, and Firefox' google toolbar offers a lot of great quick'n'dirty search tools right at my fingertips.
I'm only interested in the "Search Google", "Search Images" and "Search Dictionary" buttons on the bar
The googlebar is great if you're looking for something, and you want to find/highlight the specific word you have been searching for in the page.