I noticed a similar thing the other day in Firefox 3.6 on Windows XP. My problem was down to Firefox's zoom page function.
Steps to reproduce are:-
1) Open the compiler and create a line that is around 30 characters long
2) Move the cursor to the beginning of the line.
3) Zoom in on the page (ctrl + +, or through the main menu)
4) Move the cursor along the line of code created in 1.
5) By around character 20 the cursor is sitting in the middle of the column and deletes what appears to be the wrong character when delete/backspace is pressed.
I'm not sure this is something that can be fixed as you need to know the zoom level from Firefox to scale the character spacing for the cursor.
Looking forward to the updates :-).
Richard.
When you backspace in the compilter with FF 3.5/3.6 - It deletes the character 1 space behind the intended target. This issue is not present using IE.