I just checked, out of interest and it was "obvious" for me.
Open the web console:
"Tools > Browser Tools > Web Developer Tools"
Select the networking tab, and hit reload on your current page to get a bunch of requests. Once you do that right-click on a request and select "Edit and Resend".
The left side of the display will now show you basic details of the request, URL parameters, etc.
Scroll down and you'll see the existing headers being used (User-Agent, etc). And keep going to the bottom you'll then see "name" and "value". Those look grey, but are ticked. Enter the values you want to add, and a new "name/value" pair will appear beneath them to let you add another custom header.
Finally click "send" to make the request.
Firefox 109.0 (64-bit), running on Linux for what that's worth.
For anyone confused, I think the issue here is that the question is about response headers and the reply is about request headers. Everyone keeps saying the word header without this important modifier.
The answer above showed how to add a new header to an outgoing request:
Where the line show "Name" and "value" just enter the name and value of the header you wish to add. Those fields look greyed out, and uneditable, but they are not. When you start entering values a new row appears if you wish to add additional headers to your outgoing request.
If this is not helpful then you'll need to be much more specific in your question.
woops sorry about that. Yeah you're right, you can't actually add custom response headers, just pick from the list of response headers they give you
From looking through Bugzilla, it seems there's been quite a few tickets to add this to Firefox and some work on it was even done, but never got merged in due to some existing bugs
How do you do this on FF?