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.