How to edit or remove cookies in Firefox DevTools?

To edit cookies in the Firefox DevTools you have to ensure the Storage Inspector is enabled via the settings.

Within the Storage Inspector you'll then be able to edit existing cookies by double-clicking the values within the list:

You can also remove cookies by right-clicking them and choosing one of the options from the context menu.

Also, an option to add cookies was added in Firefox 55. To add one, either right-click the list of cookies and choose Add Item or click the + button in the panel toolbar.

I was having problems adding cookies in Firefox 65 and came here from Google.

My problem was that I could not add new cookies. I was using a Private Window. Using a normal (non-private) window however, I was able to add new cookies.