It helps to be precise. For example, what is your OS? Did you upgrade from Windows 7 to Windows 10? Did this problem start recently, like after the recent Win 10 update?
What Firefox version? Is Firefox open when this happens?
Are you getting a frozen screen, or does computer shut off? Or both? You've mentioned both, but it helps to isolate things if there is consistent response.
Laptop is Toshiba Satellite L55-B5276 (Intel Core i5, 8GB memory, 1TB HD)
OS is Win10 64-bit (upgraded in summer of 2016 from original Win8.1)
Firefox 60.0.1 (I also have IE installed and used it as an alternative to Win10, but the same freezing issue occurred wiht IE)
Problem started a few days ago. Yes, Firefox was open (except for when I trialed IE). Everything was running fine and it simply froze...the display was on the last things I'd been looking at. I could move the cursor...but I couldn't click on anything to activate...and there was a blue spinning wheel on the screen. I had to depress the power button manually for a while until the unit shut off. I've now been timing it...it typically takes 40-45 minutes before it freezes. It never shuts itself off...it only freezes.
I've been monitoring internal temps with Speccy...CPU seems to be fine...when it occasionally hits 140F, the fan kicks on and quickly brings the temp back to 120--130-ish. Storage temps are 88-90F.
I blew out the dust with canned air yesterday. A tiny amount came out....I only saw it as I had a beam of light pointed right at the computer.
I just finished opening up the laptop's case, and was surprised to see how clean the interior is. I used the canned air some more, simply as I had the case open. The dustiest area was the interior of the hinges holding up the display, and they weren't all that bad.
I tried running it with a CAT5 cable to the router in safe mode with networking , but it would not recognize the network. (Googling indicated some other folks had had a similar problem with not being able to connect when in safe mode). It does stay on longer. In fact, I aborted that after about an hour (= about 20 minutes longer than when it freezes when in normal mode).
I ran Malwarebytes and FixMyPC. They found nothing major amiss, They did a little PUP clean-up, etc.
I'm still planning running a clean install on Win10 from a live CD (per an earlier post with links <--thanks!)
I appreciate everyone's comments and suggestions. I'm learning a lot.
omni