I upgraded to Firefox 15 and got an annoying error:
I keep getting annoying often “Type error: Can’t access dead object” errors.
That message is displayed endlessly after using many web pages and closing their tab.
After some Googling I found page http://support.mozilla.org/en-US/questions/935811 that tells that Drag & Drop upload extension could be the cause.
Disabling Drag & Drop upload extension seemed to help.