Please go here https://support.mozilla.org/en-US/kb/performance-settings Performance and untick everything. change the recommended size lower then see how it runs. Note: 1 = No Multiprocessor = slow again. Try 2 Restart Firefox after making these changes please.

Note : Hardware Acceleration is for Video Card, Monitor to see if remain off or to turn back on.

Please let us know if this solved your issue or if need further assistance.

You said untick everything under Performance. The only choices available are Hardware Acceleration and Multiprocessor.

I tried Hardware Acceleration both enabled and disabled; it makes no difference -- same hanging problem.

Multiprocessor is disabled.

What do you mean by "recommended size"?

Charliesthebest said

You said untick everything under Performance. The only choices available are Hardware Acceleration and Multiprocessor. I tried Hardware Acceleration both enabled and disabled; it makes no difference -- same hanging problem. Multiprocessor is disabled. What do you mean by "recommended size"?

Please look and reread and look at the pic. Enable Multi-processor Support. With the URL provided it shows a drop down with numbers on it. That is multi-processor and you should have that enabled. As to recommended size that is for you to determine as to what works best on your system. Sometimes less is good other times more than the recommended size is good. Test....