I'm not one to usually help and/or diagnose browser problems, just an FYI.
But, I will tell you that Facebook is the slowest site I've ever experienced and it seems quite buggy too. Recently, it seems to have gotten a little better, but nothing to brag about.
Firefox often gives me "page could not load" and errors like that where I just need to refresh the page or sometimes "try again" will work.
Lately I've been using Google Chrome as my main browser, and it is very speedy. It is something you could consider changing too, or just using when you need to use Facebook. Here is the link... Google Chrome
It is very speedy, but remember, some sites don't support Chrome or like in my case, I have to use IE or Firefox to print my Paypal United States Postal Service shipping labels from, because Chrome isn't supported and it doesn't work.
Hope that is some help.