Install ModifyHeaders for Firefox:
https://addons.mozilla.org/en-US/firefox/addon/modify-headers/
Then open config panel and add your custom headers to access the REST service.
A single header has a name and a value, so it’s something like:
Name: X-Gallery-Request-Key Value: b333837c...
Add all the custom headers you need, then press “start”.
Now you can browse your REST url with firefox 