If you want to write Arabic, it’s very annoying to use left-to-right text direction, and it’s not supported natively in Thunderbird.

One of the solutions is to use the “Bidi” add-on for Thunderbird, I was able to successfully install it on my Thunderbird 8.

After installation is done and restarting Thunderbird, you’ll find two new icons for determining left-to-right or vice versa.

Update: I found it to be a very buggy add-on and sometimes views English text from right-to-left.