Usually, Firefox isolate the Flash plugin so that it can't crash the entire browser. However, that doesn't seem to be working in your case, or the problem isn't Flash itself, but something that is affecting both the Flash plugin and the browser.

When Firefox crashes, it usually records information about what was happening at that moment. You can submit that data to Mozilla and share it with forum volunteers to see whether it points to the solution. Please check the support article "Firefox Crashes" (especially the last section) for steps to get those crash IDs, and then post some of the recent ones here.

Thanks. I thought I had supplied the data along with my question. I'll see if I can attach two screen shots along with this message.

This could be a problem with hardware acceleration

• https://crash-stats.mozilla.com/report/index/9e93d019-cfa7-47d8-8e7f-b81f92140218

Try to disable hardware acceleration in Firefox (you need to close and restart Firefox).

• Firefox > Preferences > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Try to "disable the hardware acceleration" in the Flash Player.

• http://helpx.adobe.com/flash-player/kb/video-playback-issues.html

Flash "Display settings" window:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html

Thanks, but that makes no difference. Did both, restarted, and still crashed.

Incidentally, I have found videos that will crash Firefox every single time, but which are fine on Chrome.

In case any of the volunteers are gluttons for punishment, you could post links to the problem videos.

This one crashes Firefox every time...although not always in the same place. http://www.youtube.com/watch?v=8UONmuQt_98

The process is as I described above: the video goes black, the sound continues, I get the spinning beachball, and then crash.

I can watch it just fine on Chrome.

I'm not seeing the problem on my main profile (heavily customized) on Windows 7. I may need to test in a new profile later.

In case the problem is caused by one of your extensions, have you tested in Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled (Flash and other plugins still run)

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

Thanks. I tried SafeMode, got the same result: video goes black, the sound continues, I get the spinning beachball, and then crash.

Can you tell whether this was triggered by an update to Firefox, or an update to Flash, or both?

Adobe does offer a version of Flash 11.7 with security fixes which you could try in place of Flash 12.0, if a Flash update was the likely culprit. I'm not sure how you roll back Flash on the Mac platform, but the new DMG file is here:

http://www.adobe.com/support/flashplayer/downloads.html#fp11.7 ("Plugin")

I don't recall updating either before this problem started. But you know how it is - you get some random messages saying you need to update and you just click yes then forget you have done so. So maybe I did.

I tried to install the 11.7 Flash player you linked to but it wouldn't let me because the version I have installed is newer. Can you tell me how to uninstall it so I can try the older version.

I found Adobe's support article for Mac: Uninstall Flash Player | Mac OS.