When clicking on a 'link', or attempting to open another programme, I often get a message that ......."Firefox is already open in another programme and not responding, close down that programme or re-start your system" ( or words to that effect )
This problem started for me when I upgraded to 29.0.1, was a PITA so I downgraded back to 28 and everything normal again!
I'll refuse all downloads until I know they've fixed it.
NA