FEBE allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up — It will actually rebuild your extensions individually into installable .xpi files. Now you can easily synchronize your office and home browsers.

Link:
http://…/extensions/febe/febe.html

Check out these posts too:

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  3. My current Firefox extension list
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  5. What is that file extension?
  6. Trouble with extentions in Firefox v1.5?
  7. New version of the Web Developer Extension for Firefox