It happened to me with a previous 3.1x Thunderbird version. Maybe upgrade to 3.1.9. Might be some setting in html composing settings, or , I also noticed a boost when unmarked global index and search checkbox at Preferences/Advanced , General tab. Other things I did not try (as an update solved it) were deactivate grammar auto checking, use only non html composing, etc.
It might be that you have it configured as typing in text only mode, but there's a setting at composing settings, in preferences which give you several options, and default I think is type in html, rich format, if the original wrtitter was doing so. So would happen only in replies, not in a new blank message.
I am running Thunderbird 52.3 and have this problem. In fact, I've had this very problem for yeeeeeaaarrrrsssss. I've tried every solution I could find and nothing helps. I hate it. But I still prefer Thunderbird to that dreadful Outlook. – bgmCoder – 2017-11-14T01:54:52.113
Do you have any extensions installed? – random – 2011-03-23T20:13:45.810
@random: Other than the Lightning 1.0b2 that I mentioned? No. – Hollister – 2011-03-24T02:34:18.280
I got rid of lightning calendar and that seems to help for now... – None – 2011-11-09T04:56:45.450