I'm not sure whether I've seen this, but at least I haven't considered that major to file an issue for it, and I'm using Thunderbird on a daily basis.

Where this does occur, it should be addressed by #6424. Closing as duplicate.

hello.
i downloaded nvda 2017.3 just for testing and hears my observations.
i should mention that i am not familiar with thunderbird, (my test was in firefox only).
after my test, i realized that when i write some lines in edit boxes in all edit boxes, (i tested specially in gmail and wordpress websites) when i navigate in my written documents, sometimes nvda report incorrect characters, words and lines, (i see mixture of content specially when i use persian and english both in my document or only when i use persian content).
the problem for using backspace, is resolved for me many months ago, (i dont remember exactly in which version of firefox).
so, for this reason, i write my comment, messages etc in notepad plus plus, copy the content and peist in the edit boxes in mozilla firefox.
i returned to nvda 2017.2 (which is the best and my favorite version) that i decided to keep and use it forever by the Grace of God!
thanks extremely for your great screen reader, i always sincerely pray for you and request divine infinite mercy for you.