Comment 19 for bug 1970594

> [...] which I can only presume does some poor parsing of xdg-settings output (including stderr?) when determining if it is itself the default browser.

I had a look (https://searchfox.org/mozilla-central/source/browser/components/shell/nsGNOMEShellService.cpp#184-204) and it does suppress stderr, but the problem is the non-zero exit status that the error results in. Firefox will only consider itself the default browser if the command exits with 0 exit status *and* prints "yes".