We can choose font and size but the body is always LTR and we can't select the text direction.
Also, it will be great if the software recognizes the LTR/RTL language and changes the direction based on the user's keyboard language.
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Well, its 2023 and still Thunderbird doesn't have RTL support. Still usable as far as BiDi mail extension exists and works. If it stops working, I'd forget about Thunderbird. 😞