Try to use "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit) to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

Did you check for problems caused by plugins?

You can try to set the media.windows-media-foundation.enabled pref to false on the about:config page.

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

The problem occurs no matter how I close Firefox. Quit from menu, title bar close button, or otherwise.

Setting media.windows-media-foundation.enabled to false did not change anything.

I tried tracking down the device ID in the "App Notes" section of the crash log (AdapterVendorID: 0x10de, AdapterDeviceID: 0x1200) and it is my 560 GTX TI graphics card.

I switched to integrated graphics, and the crash log now lists my Intel graphics device (AdapterVendorID: 0x8086, AdapterDeviceID: 0x0412).

The rest of the crash log is identical, so I'm not sure if this is useful.

You can try to disable all plugins.

• https://support.mozilla.org/kb/Troubleshooting+plugins

Do other crash reports have a similar crash signature ([@ mozilla::ReentrantMonitor::Enter() ])?

Crash still occurs after setting all plug-ins to "Never Activate".

All crash reports have an identical crash signature. Here is another crash log with all plugins and extensions disabled.

Update: this crash only occurs when Gmail is (or has been) an open tab during the session.

• Open Gmail, close browser, crash.
• Open Gmail, close Gmail tab, close browser, crash.
• Open only non-Gmail tabs, close browser, works fine.

This only occurs with Gmails standard view. Basic view (with JavaScript disabled) does not cause this.

I tried creating a new clean profile for testing, and discovered that the Firefox firstrun page will also cause the crash. The video in the center, however, shows a "No video with supported format and MIME type found." error. On a hunch, I set media.webm.enabled to false and that page no longer caused a crash on exit.

Similarly, I discovered that setting both media.windows-media-foundation.enabled and media.ogg.enabled to false stopped Gmail from causing a crash on exit.

Also, closing Firefox before the Google chat inset finishes loading will also prevent a crash.

At a guess, it appears that Gmail (and several other sites) are trying to load HTML5 audio/video elements, and my system is not handling the decoders correctly. I can't tell if this is an issue with Firefox, or if it delegates the task to a local decoder.

This doesn't happen every time, the more tabs open the more likely it seems to happen. I have had serious problems with the minidump files in that they sometimes seem corrupt. NB this system has SSD C drive with a separate HDD for documents and pictures.

I deleted all the dump and minidump files and this crash has not occurred in last ten days. NB have also had other issues with these files in backup.

Interesting. I deleted the minidumps and the problem seems gone. Thanks!