You may have Fission enabled. Fission is about a new site isolation architecture in Firefox and you will see more Firefox processes running than before and when Fission is enabled you can no longer limit the number of content processes in Settings or via dom.ipc.processCount on the about:config page. You can check the current Fission state on the "Help -> More Troubleshooting Information" (about:support) page (search for Fission).

• https://hacks.mozilla.org/2021/05/introducing-firefox-new-site-isolation-security-architecture/

You can disable Fission by setting fission.autostart = false on the about:config page. You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

The old setting on the Settings page corresponded with the dom.ipc.processCount preference, which has a default value of 8 on most systems. Why do you want to change it?

If you need to change it due to hardware limitations, there is a "back door" way to do that:

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

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste dom.ipc.processCount and pause while the list is filtered

Make a note of the default value; do not edit this preference to exceed that value.

(3) Double-click the dom.ipc.processCount preference to display an editing field. As you recall, the valid values were 1, 2, 3, 4, 5, 6, 7 or 8. I suggest not trying any values outside that range because who knows what might happen. Press Enter or click the blue check mark button to save the change.

Probably that won't take effect until the next time you do a regular exit/restart of Firefox.