Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

From what I have been reading from other posts on this forum, the issue may be caused by the clear history settings. It was suppose to have been fixed in v30.

Type about:preferences#privacy<enter> in the address bar.

The button next to History, select Use Custom Settings.

At the bottom of the page, turn on Clear History When Firefox Closes. At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off.

This is not a cure but will make it easier if Firefox locks up in Windows. https://support.mozilla.org/questions/997866 {web link}

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button > Exit (Power button)
• Windows: File > Exit
• Mac: Firefox > Quit Firefox
• Linux: File > Quit

• "Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs

In case you use "Clear history when Firefox closes" you can try to disable this feature.