Addons and extensions are not required for card_dav. That is Google, Yahoo and GMX at least. (Use the add an address book to the right of the +New Contact button.)

Microsoft, well they are still playing with their proprietary exchange activsync.

There is a tbsync addon https://addons.thunderbird.net/en-us/thunderbird/addon/tbsync/?src=userprofile which with the help of another addon from the same guy can get exchange contacts.

Now the CSV import is what it it is and largely exists because Microsoft still offer about that and only that for a number of data interchange actions. It is never going to be slick or simple because it is one of the most basic and oldest data interchange formats out there. Plain text organized in rows, sometimes with and some times without column heading to identify what the comma, or semi colon or data separator columns should represent. Some implementations quote text other do not. Sometime the CSV will be comma separated and also contain commas in the text and not quote text so you have to manually massage the data into place.

We can add the complexity that Excel, the source of so much of peoples CSV files was incapable of reading and writing unicode text until recently and you have a mess. Excel imported and exported unicode and turned in into something unrecognizable in the process.

Largely the issues are based on the ignorance of folks about what they are doing and what they need to do to get their data into an acceptable import situation. But that is not a reason to remove a working import option.

The "drop down" is constrained that it can only use a field name once. That you found being limited to only using a field once a problem is unfortunate, but the persons email address for example can only go in the address book once. Fundamentally the best approach to CSV has always been to ensure your field identifiers in the CSV file as closely match the fields Thunderbird is expecting to import as possible. I wrote a blog post on the topic, some 10 years ago. https://thunderbirdtweaks.blogspot.com/2013/03/importing-csv-files.html It was written to support a much less forgiving user interface for CSV import. But the data setup is still important because the computer can only guess what should be what based on the field names given and there is no "intelligence" there, artificial or otherwise.

Should you wish to share your feedback or ideas with the developers rather than your peers in this support forum who really have no say in development, you might like to use the feedback and ideas link in Thunderbird's help menu. It links here https://connect.mozilla.org/

Bugs can be filed in Bugzilla. https://bugzilla.mozilla.org but i honestly do not see a bug in your posting.

The VCF import went just fine, so I'll stick with that for the time being, it would be better not to continuously import everywhere I have TB installed (I'm one of those geeks that can't multiboot enough OSes). I just wish the "big 3" Apple/MSFT/Google could standardize on something as well as MX/SMTP. Don't even get me started on lack of interop in the SMS arena...

All your suggestions are much appreciated, and I understand Thunderbird is still a Work in Progress, and needs our input to evolve. I think it's a pretty amazing product overall, with some spotty issues, but does seem to be on a great track, their first-tier support for Linux is what keeps me with the product, as well as a focus on privacy and security not found in too many email programs.