You can set the content process limit to between 1 and 8. The default is 4, but you should be able to change this.

Go to the Firefox settings and go to the Performance section. Uncheck the Use recommended performance settings option and there you will find the content process limit setting.

If that content process limit setting appears locked, go to about:config in the Firefox address bar, bypass the warning and change the value of dom.ipc.processCount to anything between 1-8.

Hope this helps.

Thank you Wesley. I am sorry to tell you that after changing dom.ipc.processCount to 2 in about:config, the content process limit remained grayed and unresponsive. although it did show the altered value of dom.ipc.processCount.

TaskManager still shows firefox.exe as a single process.

It wasn't long ago that Firefox 60 ESR ran with 4 processes regardless of the number of processor cores so something seems to have been changed in Firefox, it appears. Firefox will run 4 processes on a machine with a processor with 4 cores.

I seem to have posted two replies so have erased the second of them.

This may be a stupid question, but you have multiprocess enabled right?

To check, do the following:

1. Type about:config in the Firefox address bar
2. Bypass the security warning
3. Find browser.tabs.remote.autostart (there may be multiple)
4. Set them all to true
5. Restart Firefox.

The grayed out bits are now accessible. Back to where I thought I was. I don't remember making changes to those settings.

Anyhow, thank you very much Wesley. I would never have guessed that solution.