You can edit cookies with the "Developer Toolbar". It's under "Tools -> Web Developer" and reachable with the shortcut "Shift+F2". It's not brilliant but it works well.
Here is the entire documentation: https://developer.mozilla.org/en-US/docs/Tools/GCLI#Commands (links to list of commands, I suggest reading the entire page).
The best part is that it is extensible (Scratchpad FTW!) :)