Thunderbird has their own finances and operates quite independent from Mozilla. They make more money than any other project under Mozilla’s banner. Thunderbird is quite successful. And even if one day a problem occurs, one could still use a fork or switch to a different mail client. But I don’t see any problem coming, unlike with Firefox in example.
Thunderbird doesn’t have the same annoying stuff of Firefox, as far as I know at least. However, there is no guarantee that Mozilla wouldn’t force this on Thunderbird someday, even if Thunderbird operates mostly independent.
By switching to another client, I didn’t mean you can takeover your offline accounts and data to another client. Just meaning you can switch, as your mail accounts are not bound to any mail client. Unlike something like Photoshop in example, that was what I meant. There is fork Betterbird, in case Thunderbird decides to go wild (we can’t know that for sure). I did not look into it much, but I’m sure alternative forks that are compatible to the current Thunderbird profile (for import) will be available.