Using different computers at work and at home prompts me to look for a way to export firefox extensions from one computer to another. Currently I am using 30+ extensions @ work, but I’ve only gotten around to install a handfull @ home.
Now two extensions will let you export/backup your extensions (FEBE) and even package your extensions in .xpi files for easy sharing with others (CLEO) – go have fun
If you need proof that it works go see my firefox extensions. Here I also made a test package of my favorite firefox extensions for you to download.

History:
UPDATE 3:Ashley left wonderful information, linkting to a couple of extensions that will backup firefox themes and extensions.
Update 2: I’ve started to map all extensions and possible solutions to export firefox extensions on this page.
Update: MozBackup only supports backup for predifined extensions… The site invites users to ask for other extensions to be imported, but there’s quite a gap between the existing list to my list.
The solution would be some kind of export function for extensions. Googling leads me here , but I’d rather have a list of urls to the various extensions’ locations and then install them from their sources.
Anyhows MozBackup seems to be the best option at the moment, but as it is a program running on my laptop, I’m not too keen on that solution.
(actually, I found MozBackup via this site – which is in turkish, I think)