Cookie editing is still missing. Without this feature a wider adoption of FF dev tools or its full utility as a wedev tool is impossible.
Yes, I voted for https://bugzilla.mozilla.org/show_bug.cgi?id=1031192. Please do the same. Thanks.
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.
Nice! Thanks. New to me. Still, considering how "not brilliant" this feature is, I'd rather use Chrome dev tools when cookie editing/deleting is needed.