The Backspace key is mapped to the back arrow, but when you are in a form editing control, it should actually be interpreted as delete to the left. Maybe there is something strange about how the YT comments form works? As a short-term workaround, you can disable the mapping of Backspace to Back as follows: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter. Click the button promising to be careful. (2) In the search box above the list, type or paste '''back''' and pause while the list is filtered (3) Double-click the '''browser.backspace_action''' preference and change the value from 0 to 2 (or anything other than 0 or 1) and then click OK. (FYI - setting it to 1 causes the Backspace key to behave like Page Up which I think would be annoying in this case)

See also: *http://kb.mozillazine.org/browser.backspace_action