When I upgraded from 66.0.5 it created a new profile, rather than keeping my existing passwords/histories/extensions. I'm not going to use firefox sync, so it seems like there will be a painful migration ahead.
Docs on the profile manager are at https://support.mozilla.org/en-US/kb/profile-manager-create-...
...or, as a super weird hack, you can set the envvar SNAP_NAME=firefox to get the old behavior (see https://github.com/mozilla/nixpkgs-mozilla/issues/163 for context)