You can participate in the Thunderbird UX discussions on Topicbox (and also through email) and provide your suggestions/critiques at https://thunderbird.topicbox.com/groups/ux
And the first thing you do, is tell everyone how shit their code looks. The same code that pays for yours and everyone else's paychecks.
What do you think is going to happen?
Thunderbird is what it is; if you find it useful in its current incarnation use it (I do), but don't expect it to noticeably improve at the rate we've come to anticipate from other open-source projects. Even though Thunderbird is very popular by common open-source metrics for whatever reason it just doesn't move very quickly.
Which is an advantage, not a problem. I do not change my user behaviour regarding email often (if at all), so I really have no need or desire to see the tool I use for email changing (at all).
Every change brings with it a cost that the user must pay.
That is not an exaggeration, Outlook had them 20 years ago. People expected this feature, 20 years ago. I myself posted in the bugzilla issue 20 years ago.
I had MITMed my robot vacuum and discovered that it, like a disaffected teen, hung out in a chat room all day. I created a little python library to send it some XMPP messages so I could work around the app's terrible scheduling and put some commands in a cron file. And then I open sourced the code, because why not share?
It was a happy day for me when that robot died. Then I had a legitimate reason for archiving the project and never dealing with a user request/opinion.
It is available in GTK4 file picker. You know, major release, able to break ABI.
I think this is semantics.
Fact it, it took a decade+