Gmail is where it has always been. But switching back and forth between old, real old, and more recent versions presents a problem with user data that is saved in Firefox. "Going back" the newer files in the Profile folder can't be read and password are rendered useless. Then when "going forward" the Profile may have a hidden "switch set" and the files in the Profile won't be read properly in many cases. https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data 2019 to 2016 is a big leap backwards, and 2019 to more recent 2020 file types can present different issues. And a lot depends upon which data types are involved. Firefox version numbers (not 'years') might be the key to straighten out the mess, but can be complicated to fix. Mozilla expects and plans for updating to the next version, not "going back" especially like 3 years. Mozilla also has in place updating automatically in stages when the user "jumps forward" many, many versions, which I saw myself when going from a real old Firefox 38 version to Firefox 75 (a few months ago) with one of my Firefox installations.