Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

This could indicate that your Firefox was opted into a trial of the new Electrolysis (e10s) feature, which separates the program code from the content into separate processes. More info in this article: https://blog.mozilla.org/blog/2016/08/02/exciting-improvements-in-firefox-for-desktop-and-android/

It does not appear related to Electrolysis. I checked "about support" as recommended by article. Here is what it shows: Multiprocess Windows 3/3 (Enabled by default)

I did the safe start as indicated - no dialog box appeared, Firefox opened as it normally does.

Yes, you're on e10s. 3/3 means 3 out of 3, or all windows, using Electrolysis.

I'm not sure you can disable it, but if you want to try that:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste tabs and pause while the list is filtered

(3) Double-click the browser.tabs.remote.autostart preference to switch the value from true to false

The next time you start Firefox, it should (hopefully) return to running the UI and the content in a single process so you can test whether that makes any difference.

After several tries I managed to start Firefox in the Safe mode - dialog box appeared, etc. However, task manager still showed 2 plugin containers, with one bouncing around between 10 to 21% of CPU usage and 300 to 600 MB of memory. The other one appears and disappears, then reappears. It shows no CPU usage and 2.8 MB of memory usage. Several minutes have passed, the amount of memory reserved is now over 1,100 and continuing to increase.