Cool extension, which works in private mode! This was very important for me (private mode) and other popular extension didn't work.
So, I recommend this!

Please add an option to edit response headers as well and export and import rules so I can replace "Header Editor", I will be watching for updates

works for modifying "host:" header

Once headers is configured, its value cannot be changed. Too bad when using Authentication that changes often. Uninstalled.

Need to spoof a referer for something, and out of the six extensions I've tried this is the only one that works properly/at all.

Taking one star off for user-unfriendliness; rules can't be modified after set, they have to be scrapped and redone.

Such bad interface

Header rules can't be changed, you need to remove and add rule again. There is no export/import possibility.

It actually cannot modify the User-Agent header, if "Modify" selected. Had to click "Add" instead.

Can't edit saved settings

Works well, but for the lack of ability to edit a rule or even copy/paste the existing fields.
The fact that you can't edit a header to fix an issue makes this plugin a pain to use.
Only acceptable (compatible) headers tool add-on for Firefox Quantum.
But it could have a better UI to manipulate the headers modification rules.
Still looking for a better implementation for Firefox Quantum.

Works well, but minus 1-star for the lack of ability to edit a rule or even copy/paste the existing fields.