1911306 - general.useragent.compatMode.firefox doesn't seem to have any effect

Status: RESOLVED INVALID

(Core :: Networking: HTTP, defect)

Description

[el8]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:128.0) Gecko/20100101 Firefox/128.0

Steps to reproduce:

Changing the "general.useragent.compatMode.firefox" preference doesn't change the the "User-Agent" request header, while as far as I understand it should.

Comment 1

[Valentin Gosu [:valentin] (he/him)]

That pref does nothing in Firefox. It is only supposed to change the UA to be compatible with Firefox for other applications - like Thunderbird.
Here's the code that handles that. You'll notice that isFirefox will already be true.

https://searchfox.org/mozilla-central/rev/61ef38e43f73bc6456013bd3a1c66693727ac4c6/netwerk/protocol/http/nsHttpHandler.cpp#899-904

bool isFirefox = mAppName.EqualsLiteral("Firefox");
if (isFirefox || mCompatFirefoxEnabled) {
  // "Firefox/x.y" (compatibility) app token
  mUserAgent += ' ';
  mUserAgent += mCompatFirefox;
}