Video Auto Start. Copy/Paste about:config into Address Bar and Enter. Then Ignore. Copy/Paste media.autoplay.enabled into the top of that window in Search and wait. double-click to set it to False

See if update fixes worked : 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. Restart Firefox

Multiprocessor

You could try this please : Go the Menu then Tools --> Options --> Performance and untick everything. change the recommended size lower then see how it runs. Note: 0 = No Multi-proccesor = 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.

Setting media.autoplay.enabled to false stopped those annoying videos, thanks. Setting multiprocess to 1 (lowest) allows 4 processes to exist. Setting multiprocess to 2 allows 5 processes to exist.

The overactive caching issue remains. For instance a page like yahoo.com is always adding content, so if FF is going to try to keep that current it will quickly consume all memory. It would be better to provide a max caching threshold. I don't need it to cache stuff I haven't even clicked on yet.

I have the same version of FF on several PCs, and I don't have the high memory consumption even when visiting the same pages. Now I see one version is standard, one has a funnel cake, and the problem PC had Looking Glass (now disabled). Don't care for these spontaneous config changes. Could this be the source of different memory usage?

Very possible. Each Extension uses give or take 20megs of ram. How it uses that after it is doing it's thing can be the cause of the issue. It is also part of the Firefox processes you see in Task Manager.

Why not lets try a uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/

https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

No test this. If still having issues lets make a new Profile : https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

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

I reinstalled FF. I had refreshed recently so don't see the need. I left the performance settings at default, and noticed up to 8! processes now. On a different machine I noticed a single tab could require 5 FF processes, and a half-gig of storage. I used to routinely have 6 windows with a hundred tabs, but there is no chance of that now. It could just be due to more resource intensive sites.