OK here's what I did - the filenames only apply to me --

Navigate to /Users/ray/Library/Application\ Support/Firefox/Profiles

Notice there are default folders for multiple instances of firefox. The big one, il4dvipp.default-1544990798718, is the one for my old file name, Firefox 64.0.app; and a little one, acy9d1uj.default-release-1, is for the new copy of Firefox which launches when I rename it.  ?? that's a bug, I'd say.

Therefore:

Duplicate /Users/ray/Library/Application\ Support/Firefox/Profiles/il4dvipp.default-1544990798718

Change the name of the copy file to acy9d1uj.default-release-1

(after renaming the existing, small, acy9d1uj.default-release-1)

Now when you launch Firefox.app, it will go looking for acy9d1uj.default-release-1 and find all the 2.6 gigabytes of cookies and bookmarks and who knows what else in there.

Firefox 67 and newer use a dedicated profile for each Firefox installation and lock this profile to prevent other Firefox installations from using this profile.

• https://support.mozilla.org/en-US/kb/dedicated-profiles-firefox-installation

If you are willing to take the risk that your profile might get broken, you can use the --allow-downgrade command line switch or delete compatibility.ini in this profile folder to make is possible to use this profile.

See also "Blocking downgrades" and "Disabling the changes".

• https://support.mozilla.org/en-US/kb/understanding-depth-profile-installation