I was seeing a similar problem on two machines with Firefox and Avast.
I'm done with Avast, it's causing to many problems. I can recommend Bitdefender Free
I tried and would not recommend Bitdefender. All sorts of shoddy practices. It doesn't make a restore point when it installs, the installer had problems on my machine (hung at 73% without intervention), and when I finally did get it installed the computer would not run for 5 minutes without blue screen crashes. The blue screens could have been me hacking the installer so it would complete, but it was a horrible experience.
That's why I know about the lack of a restore point (which every piece of properly programmed Windows software should make before it installs). I managed to get the uninstaller to run in my few minutes of working system time, so I didn't have to restore from a disk image, but it was looking that way if a crash happened before I could get uninstaller to work.
Their tech support was also awful.
I would recommend Avira as a competent alternative. I also have Panda, but I didn't try to install that. AVG is now basically a different UI over Avast, so I didn't try that either.
Really, for free AV, Avast is the best software out there, and it keeps your machine running with a very minimal performance hit. I hope this issue is fixed in a future Firefox update on my machine.