1) Create a new message with two lines like this;
2) Start backspacing from on line 2 until it's completely removed, at this point another backspace should put the cursor on line1 directly after the 1. It does not but buts the cursor either at the beginning of the editor or the far right corner.
It only happens with Firefox 22, Chrome and Safari work fine.
Testing on OSX 10.8.4.