For a long time i was wondering, how to save firefox settings,bookmarks,themes and extensions as always there are chances of windows reluctant to boot up,hard disk failure etc etc. and all of your firefox hard work can be gone in a flash.A lot of my friends recommended me different applications but i myself figured out a way and i think it is the most easiest way of doing the backup.
First, go to C:Documents and SettingsyourusernameApplicationDataMozillaFirefox.There you will find

i)A profiles folder and
ii)a profiles.ini(a notepad file).
i)Inside the profiles folder you will find a folder like this.

In mine case it is cvkgefzk.default.This folder contains all your Firefox Settings,bookmarks,themes and extensions.Backup this folder on any of your other hard drives.
ii)Open up your profiles.ini file.It contains the path to my default profiles folder such as cvkgefzk.default.

So whenever you want to restore your Firefox settings,bookmarks,themes and extensions,then first you have to paste your profile folder and secondly you have to
change the name in your profiles.ini file.
And now your firefox is restored in a flash! As shiny as before.

Sanix
Great tip .. but I use foxmarks to backup my bookmarks ..
Gaz
You can save yourself a lot of work and a lot of grief by using three Firefox extensions: FEBE, CLEO and OPIE.
Febe (http://customsoftwareconsult.com/extensions/febe/febe.html) will backup your bookmarks and extensions, CLEO (http://customsoftwareconsult.com/extensions/cleo/cleo.html will pack them up in an xpi file and you can reinstall them all on a fresh Firefox. OPIE (http://customsoftwareconsult.com/extensions/opie/opie.html) will allow you to import/export extensions preferences too.
See my article on this at linux.com: http://www.linux.com/feature/125268
This was written for Linux users but it will work for Windows users too. Just select the extension versions for Windows.
Mutant
I myself know alot of extensions for doing this but the main beauty of my procedure is that your about:config settings are also saved and one doesn’t need to remember them.
Do you know, about:config ? Type it in the URL space and press Enter.
Also type about:mozilla and look what you get.
gaz
Yes, I have used about:config too (although I think the way the Opera browser handles this is much better than Firefox).
I think there is an extension for about:config too. It is called Configuration Mania. Pop over to the official Mozilla Addons site to get and and try it out (https://addons.mozilla.org/en-US/firefox/addon/4420)
jon
Firefox used to retrieve certain cookie data – eg Recent Quotes in Yahoo, favorite currencies in FXTOP for me. Firefox 3.5 has stopped doing so. IE8 manages to do it OK. Anybody know why, and how to fix?