Okay, since you want to go back to the old version, the quickest & EASIEST way is to do a System Restore back to a couple days ago. Before the new installation of Firefox. I started noticing alerts yesterday, so for those who have Firefox auto-install updates, pick a day before yesterday.
A System Restore changes only operating system stuff, like the new Firefox settings. The things it will NOT change are: document files, email, personal files, stuff like that. If you had bookmarked new favorites in Internet Explorer, from the day you pick to restore, you do lose them, unless you back up your Favorites file and then replace it later. But, that's about it.
You can always UNDO the System Restore. (Then you'd be back to the mess you have now.) For anyone not very computer savvy, it their best friend.
To do a System Restore:
Go to START -> Programs ->
Accessories -> System Tools ->
System Restore
Then pick a date like last Wed, what ever is BOLDED around then. (Remember you can always UNDO it, or even Undo and pick an earlier date if necessary.)
It will reboot you to an earlier time and you should have the older version of Firefox in place.
Once your computer is back on:
Open Firefox.
Click on the Help in the grey bar at the top.
Click About Mozilla Firefox. It should tell you you have version 3.6.12 (or earlier.) Close the box.
Immediately click on Tools
Click Options, then Advanced.
Click the Update.
Make sure the button for "Automatically download & install the latest update is NOT checked."
You'll get a little grey box pop-up once a day,

then a bigger grey box asking you to install the newer version again. Simply click, "Remind me later."
It will keep doing this every day.

But, sooner or later, it will give you a
different updated version -probably the one that fixes
this Facebook problem. Which is why I don't want you to turn off the daily alerts. Later on, you will probably be able to install whatever update that fixes this problem.
One time I was so lazy getting around to install the latest version, I
missed a whole version update! So there is no problem if you don't install this new one.