Make sure you have thunderbird set to show simple html, this will prevent several issues with spammers, but will also allow NVDA to read things a bit better.

With the full blown html, there are things that NVDA doesn't handle well, though it's rarely NVDA's fault, it's the html code that folks haven't a clue how to writ themselves, so they use programs to generate it for them, and some programs don't do a very good job of generating html code that is standards compliant.
This may not solve your problem entirely, but it probably will help.

Hi
I can no longer get NVDA to read my HTML emails in Thunderbird one line at a time. This means it is impossible for me to edit parts of the email. It will only read the email from start to finish.  It is a HTML table with several rows. It won't even read one character at a time, only the whole table.
May be a very simple answer but I am not a tech genius and cannot find an answer in the user guide. As I am running a business from home this issue is causing great frustration. Worked perfectly fine in the past and fine in plain text emails.
Not sure if issues started with latest update of NVDA as have auto update on, same with Thunderbird, so all current versions installed. Using desktop with Windows 10.
Or it may have changed as every so often NVDA crashes and resets to factory defaults. And will not re install my saved profile as well for some reason and as so many preferences I cannot remember them all! So perhaps just needs the right option ticked in settings.  But which one?
Please help, thanks
Justine