Is it closing, crashing, freezing . . . . . . . Were you actively using the browser at the time?

At 1420; Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

In the address bar, type about:crashes<enter>. Note: If any reports do not have BP in front of the numbers/letters, click it to submit them.

The crash report is several pages of data. We need the report numbers to see the whole report.

Using your mouse, mark the most resent 7 - 10 crash reports, and copy them. Now go to the reply box below and paste them in.

Any chance that security software is doing updates at that time everyday? The time setting for updates can usually be changed to a time of day when the computer is expected to be idle; as long as that PC is left running.

Same for any other programs that might do timed data transfer - up or down.

I have had a problem with any web browser running very slow at the same time everyday due to anti-virus database updates at the same time everyday; tweak the time setting in the AV program and no more problem.

But I never had Firefox actually crash, just run slower than hell as all my bandwidth was being used by the AV program. Wait a few minutes and the problem is gone.