One should never, and I mean never, expect that beta versions of any software are going to be completely bug free, and all the more so if one thing (Thunderbird, in this case) is a beta that's significantly in advance of the stable software (NVDA, in this case) it's being used with.
Reports of these issues are very valuable, but you really can't expect that anyone will devote a lot of time to them until the release candidate stage is reached unless it happens to be convenient. Definitely give the feedback to the folks at Mozilla just like you did here.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Power is being told you're not loved and not being destroyed by it.