FEBE (Firefox Environment Backup Extension) is a cool Firefox addon, that enables you to backup your Firefox extensions. It will actually rebuild your extensions individually into installable .xpi files, that enables you to synchronize between office and home PC.
This extension is available for Firefox 3.x.
As claimed by the addon description, FEBE backs up your extensions, themes, and (optionally) your bookmarks, preferences, passwords, cookies and just about everything else Firefox offers (it can even backup/restore your entire profile).
Another nice feature, is that it enables backing up directly to your Box account. This facilitates easy sharing and portability.