I see, there is a problem with using "about:manual" in our base-browser patches. We have the same problem with the security level "Learn more" which leads to about:manual#security-settings.
I wonder if we should set them to https://tb-manual.torproject.org/ in base-browser, and then switch them to "about:manual" in tor-browser.
Similar to how we use mozilla support links in Tor Browser, I think it would be ok for mullvad browser to link to https://tb-manual.torproject.org/.
In the case of letterboxing though, I don't think we have a manual page, so I'm not sure were "about:manual#letterboxing" is meant to lead to. @ma1 is this a planned page for the 13.5 release? The only thing I found at the moment is https://support.torproject.org/tbb/maximized-torbrowser-window/.
I've got a crazy idea for this: patching AboutRedirector::GetManualChromeURI() to fallback on tb-manual.tpo (or possibly another URL which might be set in a pref, which would make this skippable if we want?) when no chrome mapping is found?