How do I delete a cookie in Firefox Developer Edition?
(Not a good sign for this product that this question needs to be asked).
I'm using v35.0a2 (2014-11-14) on Windows.
Remove button does not seem to work on Mac version :-/
click the "remove" button
new code appears on the command line
click the Return key on your Mac
and the cookie is gone
If your site has more than one cookie or you are doing this a lot because you are developing something, consider getting a plugin to reduce the number of steps to one.