My guess, based on both things now said, is that this is likely more on the Thunderbird side of things than NVDA, per se.
A screen reader only reads text (or alt-text, for graphics) that is exposed to it to read. It almost sounds like it's now reading the character used for many things as far as expanded or collapsed or dropdown indication goes. That downward pointing small triangle sounds like the conversation expanded indicator in the Tbird inbox for a given conversation you've landed on. I'd expect that something very similar, but probably saying right pointing, is said if it's collapsed.
It could be an NVDA thing, but since this sort of symbol has been present "since dirt" in Thunderbird, and the announcement of expanded or collapsed had been, too, I suspect something's creeped in on the Tbird side.
Are either of you still getting expanded/collapsed announcements for conversations or in the folder tree? Or is that weird triangle announcement being said everywhere, or perhaps only in, say, a mailbox list of messages but not the folder tree?
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
Any idiot can face a crisis. It's the day-to-day living that wears you out.
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