Real minor version is exposed in MB's non-RFP HTTP User-Agent header

[Jee-Hex]

Like Thunderbird, MB is currently exposing its real minor version in its HTTP User-Agent header whenever uBO initiates outgoing connections for filter lists updates, since RFP stopped affecting WebExtensions a while ago. If you visit moz-extension://[ublock-origin-webext-uuid]/dashboard.html#support.html in MB 14.0.3/14.5a1, you can see that uBO is reporting Firefox's real major version as 115, even though 14.0.3/14.5a1 is based on 128.5.0esr. I think this is because MB is failing uBO's check in https://github.com/gorhill/uBlock/blob/1.61.2/platform/common/vapi-common.js#L186-L187, since it's actual non-RFP User-Agent doesn't come with a Firefox/. And that in turn is probably due to the usage of export MOZ_APP_BASENAME=MullvadBrowser in https://gitlab.torproject.org/tpo/applications/mullvad-browser/-/issues/75.

Looking at https://searchfox.org/mozilla-esr128/source/netwerk/protocol/http/nsHttpHandler.cpp#391-392,863-874 (or bug 728831), it would appear that Firefox will only trim off the minor version and patch level in the Firefox/ part of its User-Agent. One way to "fix" this is to simultaneously remove https://searchfox.org/mozilla-esr128/source/netwerk/protocol/http/nsHttpHandler.cpp#869-875 and lock general.useragent.compatMode.firefox to true.

[PieroV]

Hi, thank you for the report and for the suggestions.
I'm going to open an issue on our GitLab and follow there.

EDIT: https://gitlab.torproject.org/tpo/applications/tor-browser/-/issues/43386