What exactly happens and do you get any error messages ?

Is it the whole computer that shuts down or just the Firefox browser ?

Quite a few possible reasons but two are

1. The computer overheats.
2. You run out of memory

If your computer has a variable speed fan you will probably note that running at high speed, if you are in danger of overheating. Software is available that will monitor and check temperatures. Dust build up or obstructed ventilation are possible causes, & probably noticeable if watching videos.

Firefox or the computer may crash in some circumstances if there is no more available RAM memory or HDD space. That is probably more likely to cause a Firefox crash rather than the whole computer crashing.

No error mesage it just isn't there in the morning. The computer is not hot and I have not run out of memory. This just started about a week ago. Outlook Express and Mozilla are the only things running.

Have you installed any privacy-related software that might be shutting down Firefox so that it can remove history or cookies??

Nothing was installed or changed thats whats so confusing

Why not shut down the browser when you are not using it? I presume you sleep sometime. You might find it a bit more responsive that way.

If you are leaving it overnight have you considered power management or screen savers, or possibly some task such as malware scans or updates that run when idle.

It is possible that Firefox has crashed.

If you have submitted crash reports then please post the IDs of one or more recent crash reports that have a "bp-" prefix:

• bp-xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

You can find the IDs of the submitted crash reports on the about:crashes page.

• You can open the about:crashes page via the location bar, like you open a website, or open the crash reports page via "Help > Troubleshooting Information".

See:

• http://kb.mozillazine.org/Mozilla_Crash_Reporter
• https://support.mozilla.org/kb/Mozilla+Crash+Reporter