The solution, as pointed out by others on Github is to replace AccessibleHandler.dll in the application directory with the dll from an older version. For convenience, I've attached the affected dll from 2020.09.
After replacing it, the option to toggle multiprocess mode may still be greyed out. If so, create the following boolean in about:config
browser.tabs.remote.force-enable
and set it to true
It should now be enabled after restarting the browser. Simply uncheck Recommended Performance Settings and choose the number of content processes to your liking
Remember to disable automatic updates!
I advise creating a scratch profile to test this on first, to avoid potential data loss in case of complications. It shouldn't be an issue, but just in case.....
(tested with Wf Classic 2021.03 on 7 x64)
