Hello,

Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

I agree with the last post, Firefox's safemode is worth trying. For instance that disables hardware acceleration, and even with a clean reinstall and clean profile hardware acceleration is on by default.

You mentioned by PM you have problem on a specific machine but not other machines so at lest that narrows it down to something that differs between machines. Graphics drivers and hardware acceleration may be involved.

Remember to disable all plugins when testing as even in Firefox's safe mode plugins are not necessarily disabled. Trying Windows safemode is also worthwhile.

Is Firefox hanging on the first click and so not necessarily on shutdown if so the built in profiler may help. I doubt I would understand the output but we will be able to file a bug or find someone who does. Even without understanding the output fully it my throw up clues as to what is happening.

• https://developer.mozilla.org/en-US/docs/Mozilla/Performance/Profiling_with_the_Built-in_Profiler

Thank you. I will give it a try. I am currently not in front of that machine. Concerning add-ins: I had already erased absolutely everything with "Mozilla" on it from that machine before to make a virgin install because add-ins were of course my first suspicion.

After doing more clean up, I got a freshly installed 36.0.1 to work again.

The error re-emerged when I tried to sync with by Firefox profile again.

I use the same profile on a Win8 and an XP machine where it works without problems.

Working with clean profile and a clean install presumably you will be able to reproduce this by signing in to Sync then clicking on a link.

If this is down to a Sync problem maybe it shows up in the sync logs, so after seeing the problem and restarting try keying into the address bar about:sync-log I think it logs errors by default. Do you have such error logs ?

I know Firefox is now supposed to always close down. If it fails to close after about a minute it should actually Crash and generate a Crash Report. If Firefox does not crash it should be possible to deliberately Crash Firefox whilst it is hung. There is a utility to do this downloadable from https://ftp.mozilla.org/pub/mozilla.org/utilities/crashfirefox-intentionally/crashfirefox.exe

Thank you for all your support. Given the rapid succession of Firefox releases, I just waited for 37 to come out. My problem is gone now.