With the changes to updates it not possible to revert back unless you install older version over the current version. If you want to get firefox back it's best to backup your bookmark and uninstall all firefox and deleting the mozilla folder and reinstalled a the latest version that way it's a clean install and avoids any update pitfalls.

https://ftp.mozilla.org/pub/firefox/releases/

You can check the performance settings and set a lower count for content processes.

• https://support.mozilla.org/en-US/kb/performance-settings
• Options/Preferences -> General -> Performance
  remove checkmark: [ ] "Use recommended performance settings"

You can find the current multi-process state on the Troubleshooting Information (about:support) page.

• "Help -> Troubleshooting Information" -> "Application Basics":
  Multiprocess Windows
  Web Content Processes

benmillan said

Firefox has been opening multiple processes and is slowing down my computer.

Hello benmillan,

In case you have multi process (e10s) enabled, you could try this :

Type in the address bar about:config and press Enter (promise to be careful, if asked)

Type in the search bar and look for the preference :

browser.tabs.remote.autostart

and set its value to false

Then close and restart Firefox.

Looks like that worked! Thanks! : D

Did you try how it works for you with only one content process?

• https://support.mozilla.org/en-US/kb/performance-settings

This is meant as a feature to protect against cases where a tab crashes. With multi-process completely disabled like you did with setting browser.tabs.remote.autostart to false then Firefox will crash when one tab crashes.