A bit of playing has revealed that the Read indicator is strictly tied to an individual message, not a thread/conversation, and will only be accurate for an individual message within a conversation.
However, there is a separate column (much closer to the end of the column list) entitled Unread, which is not an on/off indicator, but a number that shows how many messages in a conversation/thread are actually unread. It shows nothing (not zero, but nothing at all) if there are no unread messages in a thread/conversation.
You may want to use Mozilla Apps Enhancements to move the Unread column somewhere in the first several columns on display, so you will hear the number of unread messages in a conversation when such exist. Then, when you are moving through the messages in the conversation the Read/Unread indicator will be active/on when the message itself is unread and be turned off after you've read it.
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Brian - Windows 10, 64-Bit, Version 21H2, Build 19044
Nothing in all the world is more dangerous than sincere ignorance and conscientious stupidity.
~ Martin Luther King, Jr.