Please : Go to the Firefox 3 Bar Menu --> Help ? --> Troubleshooting Information Page and take a look in the Accessibility section if accessibility is set to "true" there. if yes, go to the Firefox 3 Bar Menu --> Options --> Privacy & Security panel and under Permissions check the setting to Prevent Accessibility Services from accessing your browser.

Monitor in Task Manager. Should stay below 2 gigs. If excessive then do below and monitor and change if need to.

Go the 3 Bar Menu then Options --> General --> 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.

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

I already tried these first steps regarding accessibility. I just tried again, but no difference.

Memory usage tops out at about 300 MB, so not too unusual.

That is wonderful. I run at just under 2gigs. I have ram to spare though. But the expected is around 1500 I think with multiple tabs open, each Extension uses give or take 20megs which is included in Task Manager Firefox Icons. Go to some heavy HTML5 audio or video or other and it will climb. https://www.howtogeek.com/334594/stop-complaining-that-your-browser-uses-lots-of-ram-its-a-good-thing/

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

Well, everything seems to be working normally this morning. The only thing that changed is that Windows 10 installed the following updates overnight: KB4053577 KB4058043 KB4054517 I suppose it must have been some incompatibility that was addressed by those updates. Thanks for your efforts Pkshadow.