The upgrade to Firefox 4.4, aka Firefox 7, has just been released. Warning: if you use AdBlock, it will disappear when you install the upgrade.
However: you need to install the upgrade anyway.
Allow it to install and restart Firefox. DON'T PANIC. Once Firefox is open again, go to Tools --> Add-ons --> Get Add-ons, and type 'AdBlock' in the Search window. Reinstall it. Hurrah! It will reappear when you restart Firefox, looking none the worse for its brief absence.
If you use Firefox and you are not using the AdBlock add-in, OMG WHY NOT??? Come! Join us in [nearly] ad-free nirvana!!
If you use Internet Explorer, um, WHY?!?
If you use Chrome, none of this applies to you anyway, you smug bastion of nerditude.
I'm hoping that Mozilla, or the makers of AdBlock, will tweak this within the next day so it no longer happens. But if it's already happened to you, fear not. Recovery is breathtakingly easy, as described above.
ETA: They did, in fact, fix it within 12 hours. Yay.