Make sure that popups are blocked, scan for malware, and maybe also install Adblock Plus. Doing those things should cut down on the number of annoyances.

I was having this problem (every time I opened Firefox, multiple tabs of Facebook kept opening, even when I was on other websites such as Google).

Having restarted Firefox with add-ons disabled (via Help in the Firefox menu), I enabled first the Plugins and then the Extensions, on-by-one.

The problem only recurred when I enabled Facebook Toolbar 1.7.3 and vanished when I disabled it again.

This extension was last updated on 3rd August, 2011, which is probably when the problem began to manifest (growing steadily worse). The extension tries to integrate certain Facebook utilities with the browser but this is something I can easily live without. If you are having similar problems, try disabling Facebook Toolbar 1.7.3