If you know you have a configuration profile set up for Thunderbird, check that the volume under the speech settings is not "maxxed out" or near to it for NVDA.
If you don't think you do or should have a configuration profile for Thunderbird, still check under Configuration Profiles in the NVDA Menu (NVDA + N, C) to check if somehow one got created.
That would be my first theory as to how this could occur.
Brian - Windows 10, 64-Bit, Version 21H1, Build 19043
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