Firefox: See what modified my HTTP headers

Question

How can I find out what part of my Firefox installation modifies my HTTP headers?

Using a tool that displays my headers for the corresponding request I can see my headers contain the following string:

Accept-Language: de,ar-SA;q=0.8,en-US;q=0.5,en;q=0.3

I want to find out how ar-SA got in there.

Answer 1

I don't know how you'd go about seeing what changed your HTTP headers specifically, but I do know where you set your preferred languages. In Preferences, go to the Language section:

And then click on "Choose..." to see them:

I added "Dutch [nl]" to mine to see if my headers would change and sure enough they did:

Accept-Language: nl,en-GB;q=0.7,en;q=0.3