Browsers send HTTP Accept headers. This is an example of one:
text/html, */*; q=0.01 gzip, deflate en-US,en;q=0.8,sl;q=0.6
You can get yours here.
How can I change curent headers from terminal app for Chrome or Firefox?
Forefox has a collection(dictionary) of
user_pref's you can set in
prefs.js. However you first need to know the correct entry you are setting. Is there a list of all entries developer can set?