My new favorite Firefox extension is "Firefox Environment Backup Extension" or FEBE. My work machine, still a desktop, is my most finely tuned box. My home machine is always lagging behind in Firefox (and Emacs) updates. I have often wished for something like FEBE and now its here. FEBE allows you to export all your Firefox extensions and settings and import them into another computer. A nice addition to the thousands of Firefox extensions.
I'm sure Internet Explorer 8 has something similar since the latest Microsoft sponsored browser "Get The Facts" report gives IE8 and Firefox both a check for customizability. (Can somebody tell me why the delicious tags on Microsoft's "Get The Facts For Browsers" page are "funny" and "humor"?)