Recently Facebook has been running very slow for me and even just hanging completely on page reload. I wasted a lot of time trying things within Facebook and after reading of many others with similar problems was even considered abandoning Firefox as my preferred browser, but as soon as I disabled the Skype Firefox extension everything returned to normal. So if you are having similar problems, this may be the solution. (The extension is not necessary for the normal use of Skype – just for having phone numbers on web pages all become clickable.)
What disappoints me about Skype as a result of this is not the fact of the problem per se (making different programs interact is often difficult and it is not surprising that things go wrong), but rather the fact that there has been no loud public warning from Skype of a problem which has been around for some time (in fact the extension is still loaded automatically without warning during Skype upgrades). If Skype had acted appropriately there would have been a high ranking Google hit which resolved the issue but in fact on Googling Facebook and Firefox I found many discussions which did not identify Skype as the source of the problem, and there has been no warning transmitted during several recent Skype upgrades.