Working regularly on at least 4 computers (at home and work), synchronizing the passwords, extensions and bookmarks between them is a daunting task. However there are 3 extensions that help you manage this task
Google Browser Sync for Firefox is an extension that continuously synchronizes your browser settings – including bookmarks, history, persistent cookies, and saved passwords – across your computers. It also allows you to restore open tabs and windows across different machines and browser sessions.
Firefox Extension Backup Extension 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.
FEBE backs up your extensions, themes, and (optionally) your bookmarks, preferences and cookies.
Another great extension is Compact Library Extension Organizer. CLEO is a Firefox extension that works with FEBE to package any number of extensions/themes into a single, installable .xpi file.
(Thanks for Blot for the heads up on the last two)
For more powerfuls extension check out our Most Useful Firefox Extensions page