It rarely happens with me.. If it took a long time, try terminating it manually from '''Task manager''' and it would work again.

From what I have been reading from other posts on this forum, the issue may be caused by the clear history settings. Press the '''<Alt>''' or '''<F10>''' key to bring up the tool bar. Then '''Tools > Options > Privacy'''. 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. '''https://support.mozilla.org/questions/997866?esab=a&s=&r=1&as=s''' {web link}