Please see my thread re the very same issue............
http://forum.avast.com/index.php?topic=150381.0Firefox 29.01 did not fix the problem, in fact my problem arose when Firefox 29.01 updated from Firefox 29
I'm on windows 7 64bit, Avast free
The only way I could resolve was to roll back to firefox 28.........which worked again perfectly, and then this updated directly to 29.01............
I won't go all into my workarounds here, they are in the link of the thread I posted, which I hope might be useful to people. It is very long winded though, and wanted to comment that though it is thought and said to have been fixed, from my recent experience, it isn't just yet..........
I'm glad more people have posted about this.
Surfseek