You can try to revert MicroB back to defaults and see if it still occurs. To do so, close all browser instances start xterm and type:

After the reboot MicroB will rebuild with its default settings cleared of all user data. If the browser still behaves as it shouldn't (I haven't really noticed, I don't use FB and I don't have problems on other sites), you can report a bug at Bugzilla and revert back your original settings by closing all browser instances again and typing in xterm:

After the reboot MicroB should be as it was prior to first 'reverting' to defaults.

If the MicroB behaves nicely after clearing it up, but you don't want to loose your settings, saved passwords and etc. you can still revert back to the original settings and live with it, but if you want to keep using like that you can remove the backup of the old settings previously created by typing in xterm:

Hope it helps...