First,
Type about:preferences#advanced<enter> in the address bar.
Under Advanced, Select Update.
[v57+] Type about:preferences#general<enter> in the address bar.
Select Update.
Select Never Check For Updates.
Also turn off Use a background service to install updates
Install Older Version Of Firefox {web link}
Be sure to read everything here.
If you still want to downgrade, look under;
I still want to downgrade.
Click the Directory of other versions and languages link.
Look for the directory of the version that you want.
But, remember that old versions may have security issues.
You should also check out
Firefox; Extended Support Release {web link}
ESR Notes
System Requirements
First,
Type '''about:preferences#advanced'''<enter> in the address bar.
Under '''Advanced, ''' Select '''Update. '''
[v57+] Type '''about:preferences#general'''<enter> in the address bar.
Select '''Update. '''
Select '''Never Check For Updates.'''
Also turn off '''Use a background service to install updates'''
---------------
[https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Install Older Version Of Firefox] {web link}
Be sure to read everything here.
If you still want to downgrade, look under;
'''I still want to downgrade. '''
Click the '''Directory of other versions and languages''' link.
Look for the directory of the version that you want.
But, remember that '''old versions may have security issues. '''
You should also check out
[https://www.mozilla.org/en-US/firefox/organizations/ Firefox; Extended Support Release] {web link}
[https://www.mozilla.org/en-US/firefox/organizations/notes/ ESR Notes]
[https://www.mozilla.org/en-US/firefox/organizations/system-requirements/ System Requirements]