User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0 Build ID: 20160604131506 Steps to reproduce: 1. Open new Message Compose Window. 2. Type a few words (e.g. "Hello Peter,"). 3. Hit enter twice. 4. Start a sentence (e.g. "This is a"). 5. Mark all three lines and change the paragraph format to "body text" (was originally "paragraph"). 6. Continue the sentence (e.g. " Test."). Actual results: After clicking at the end of the unfinished the sentence, the caret is positioned correctly. As soon as I start typing, the caret is moved to the line above and the resulting mail looks like this: Hello Peter, Test. This is a Expected results: The caret should not have moved and the mail should look like this: Hello Peter, This is a Test.
This bug can still be reproduced in the nightly build of the 50 branch from 7th July 2016.
We know this problem, it has already been reported and analysed in bug 1281664.