(Like many of my technology postings, this is really here for my own notes on problems I run into, the solutions I try and what works when. They are updated as I learn.)
To disable multi-process Firefox, do the following
Type about:config in the browser’s address bar.
Confirm that you will be careful.
Search for browser.tabs.remote.autostart.
Double-click on the preference. It should change from true to false. Restart the browser.