This is the primary repository for every Firefox release ever made. You can find them at ftp.mozilla.org/pub/firefox/releases/ .

Run the installer you downloaded in Method 1.

Look for your regional code (e.g., en-US for English-US).

If you do not do this, Firefox will immediately update itself to the latest version the moment you open it.

Downgrading can sometimes corrupt your profile. Back up your bookmarks and passwords first.