When you have your Firefox set up the way you want with all your extensions, bookmarks, saved passwords, etc. You might want to migrate that same setup to other computers.
Disclaimer: This isn't what's recommended by Mozilla, it's just what worked for me.
* Install Firefox on Vista or Windows 7, launch it once, then close it. Make sure Firefox on XP is closed also. Important! Make sure they are the same versions! You cannot migrate between different versions!
* Go to XP: Back up C:\Documents and Settings\<username>\Application Data\Mozilla\Firefox\Profiles. It should contain only one folder, which has a garbage name like "9cofchse.default" or some such nonsense.
* Hit Ctrl-C to copy that entire folder.
* On Vista, go to C:\Users\<username>\AppData. Now go in to the "Roaming" folder, then Mozilla, Firefox, Profiles to get to C:\Users\<username>\AppData\Roaming\Mozilla\Firefo x\Profiles. There will be another folder there with a garbage name.
* Hit Ctrl-V to paste the folder from the XP Firefox to this folder next to the other folder with the garbage name.
* Now go to the other folder in this area and select it. Write down that folder name. Then Press F2 as if you are going to change the file name. But when the filename is highlighted, press Ctrl-C again to copy that filename. While it is still selected, rename it to "default.profile" and press enter to change the name.
* Go back to the folder you just pasted in and change its name to the same as the folder name you wrote down (that you just copied to the clipboard).
* You have just replaced the default folder in the new Firefox location to the same as your other XP profile.
* close all the windows and review your steps.
Now all you have to do is start Firefox and pray. If you did it correctly your new Vista or Windows 7 Firefox should have everything that your old XP Firefox had.
Edit: I am aware that the mozbackup extension can do this as well. But I figure why load an extension in memory for a function that you can perform once a year by hand. Whenever you have to back up your profile, just backup that profile folder... it has everything you need!