Implement better fix for Torbutton signing workaround

In legacy/trac#27271 (moved) we implemented a workaround for mobile related to the Torbutton signing exemption. We should think about a cleaner fix as mentioned in comment:7:ticket:27271.

There I noted

FWIW: ​https://bugzilla.mozilla.org/show_bug.cgi?id=1464766 landed in esr60 and might be a thing to consider here.