Do not default to a new, emtpy profile if profile import is possible

[Zurdo]

When running an Older thunderbird version and reading a default profile from a Newer version (which cannot be directly loaded), consider immediately, or more obviously, suggesting to import the profile into the Older version, which is otherwise possible manually, instead of opening into a blank, new profile.

(Are there any circumstances where Thunderbird Stable is opened and finds a Beta profile as default where the most likely scenario isn't a rollback/testing attempt which would benefit from this?)

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Context: I've just had a snafu when using Thunderbird Beta. Since the most reasonable way available for me to fix it was rolling back to the previous version, I reached for my backups, but not without first going for a hail-mary, in installing the current stable release.

When running it, I got a warning that the default profile was too new, with a button to create a new profile, which I did.

The fun came when, by pure chance, I thought to use the import function, which I didn't really know could import thunderbird profiles (I've been carrying the same profile over for a while, manually editing installs.ini and profiles.ini when necessary). Not only did this work (yay), but it didn't even complain about the profile being from a newer version. Except for passwords, the default search engine, and spelling dictionaries, everything seems correctly imported (pending user.js and config.js revision, tbf)

This exceeds any expectation I had; I was fully prepared to recover a backup of both the beta build I was running up to now and the profile. But given the program already had access to enough information to suggest recovering the newer profile, I feel offering this functionality I didn't know thunderbird had would have been less spooky.

[Jon]

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