Okay, XMarks (formerly FoxMarks) does sync your bookmarks across multiple computers, and it also works with different browsers. But, if you use only Firefox on all your computers (you should consider joining the Spread Firefox community), then you may use the more integrated Firefox Sync (formerly Weave Sync), which is about to be baked into the next Firefox.
The problem: none of these add-ons sync your add-ons. You can have your bookmarks, history, passwords on all your computers, but if you have a ton of extensions, you have to install them manually on all your computers. Now that's boring. Siphon to the rescue!
Yes, if you remember, Siphon is the antagonist in Race to Witch Mountain, but it is also a less-heard-of Firefox extension. You just install it, make an account and sync. Install it on other PCs, log in, and it will download and install all your extensions automatically and install them for you.
Download Siphon here. It is a beta for now, so you may have a few problems, but I like it anyway, and I don't find it too buggy.