You are not alone:

• https://crash-stats.mozilla.com/repor.../7493278c-dc37-439c-ba3b-9f3ea2150723

When looking at the report and clicking on More Reports, there are others with the same OS that this is happening it. I can suggest two things from this information: check to make sure that there are no outstanding updates for the operating system as well as try to disable hardware acceleration in the video player that you are watching videos from. (?Flash)

Thanks for the advice. I updated everything and and disabled hardware acceleration and Youtube still crashes. Curious.... and frustrating!

This is not a cure, but should let the web page load without crashing.

Flash block {web link} Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on it

Thanks for the suggestion! I had already put Flash as "Ask to activate" in my Add-Ons. How is this different?

Hmmm.. So I installed Flash block and I still get crashes. So maybe my problem isn't Flash?

MimiF said

It crashes exclusively when video content is launched, especially youtube. But not systematically. About 7 out of 10 times. Starting Firefox in Safe Mode did not change anything. Last crash report ID number : 7493278c-dc37-439c-ba3b-9f3ea2150723 Mac OS is 10.8.5 Thank you!!!!

With me I don't get a crash report so I haven't any ID number to help, but when I close FF and start back up I get this: (Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window.) I click ok and sometimes FF will start up with out a problem and sometimes I need to reboot to flush it out. Gerry

Try to disable the HTML5 media player by toggling the MP4 pref and possibly the MP3 pref as well if the problem stays.

• media.apple.mp4.enabled = false
• media.apple.mp3.enabled = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config