Using Thunderbird (edge) as an upgrade to the default version fails, because the snap uses a non-standard location for the user-profile. Any existing profile will be stored in ~/.thunderbird but snap uses ~/snap/thunderbird/common/.thunderbird/
The official way to tell Thunderbird to use an older, existing profile would be entering thunderbird -ProfileManager in a console, but the profile manager has no ability to look for a profile in a different directory.
You can try just copying (or moving) the entire existing thunderbird profile from ~/.thunderbird to ~/snap/thunderbird/common and you should be good. Pretty sure I did this when I migrated from the traditionally installed thunderbird to the snap.
Well, I migrated FF and TB to snaps. Seems to work very well. Still the first run quite slow (using i7+NVMe…), but you know that.
Do you know why LibreOffice or Snap Store snaps still do not render my fonts correctly (Roboto from Ubuntu repo) ? Because fonts are fully ok in FF and TB snaps (spacing is as in deb versions). I do use Wayland GNOME session.
Last question : since some deb versions, FF does not respect default UI font in search bar. But I notice it’s ok using the snap. Why ? (Please note this snap better behaviour than deb for future use. )
Last last question : will TB be a Canonical (as author) snap ? I know Kevin is yours, but Canonical is not a one-man-team and FF snap is from Canonical.
Currently experiencing the same for Thunderbird 91 (candidate).
I believe this happened after the update because I remember checking them earlier this week (last update at the time of writing is August 27).
I checked ~/snap/thunderbird/common/.thunderbird/, and it created a new profile. My old profile is still there, and still accessible when I ran snap run thunderbird -ProfileManager