Hello NVDA users,
As requested by moderators, please direct further comments to GitHub issue:
A few minutes ago one of the NVDA code contributors tweeted that annoying status bar announcement issue is caused by changes made to Thunderbird itself. In short, Thunderbird placed a tag on its status bar that will cause NVDA to announce messages continuously (technical: ARIA role status was used for status bar).
Unless it is justified that NVDA should resolve this issue on its own, this issue is something Thunderbird developers should resolve.