What's interesting to me is that I actually like the old interface better than the current one. But the part that matters, the actual email list, has barely changed in 20 years. This is the place where I feel they need to modernize most. Thunderbird needs a robust, clean conversation view, like GMail or macOS Mail.app (That app is actually one of the main reasons I use a Mac right now. If there was an on par OSS version I would probably use Linux.)
Also, I miss proper Outlook (Office365/Exchange) integration. I hate it, but if you use it at work it has to work. There is an integration, but it is a paid plugin.
That said, I really hope Thunderbird succeeds and becomes relevant again. If you have a certain amount of mail, a real mail client is just so much better than webmail, and it is great to have an OSS version.
Also, I miss proper Outlook (Office365/Exchange) integration. I hate it, but if you use it at work it has to work. There is an integration, but it is a paid plugin.
That said, I really hope Thunderbird succeeds and becomes relevant again. If you have a certain amount of mail, a real mail client is just so much better than webmail, and it is great to have an OSS version.