Thanks for all responses.
Perhaps I should clarify. The HTML emails are templates and I need to change name and price and details each time Want to send them to a new customer. They have worked fine until recently when they have become impossible to edit because NVDA reads the whole table from start to finish.
I have tried NVDA and space bar to change to browse mode but that it is not possible now either. Although works fine with rest of Thunderbird.
View message body is only an menu option in received emails and not ones I create from new or templates. In received emails it is set to original html Change to simple HTML and made no difference.
When i want to edit my template I select the line I want to alter and silence, if I arrow down to the next line it will only start reading from the beginning of the table and won't stop till it reaches the end. If I interrupt it, by arrowing up or down it starts reading from the beginning all over again!
Playing around, I have looked at the email in my sent folder and NVDA reads this one line at a time as it should.
If I reply to this email and am right back where I started and reads the table from start to finish.
Someone suggested it may be the HTML code but it worked perfectly for the last year until recently!