Comment 22 for bug 1827727

(In reply to Milos from comment #16)

> Can somehow signing be disabled?

I don't think so if you are using Firefox 48+ on PC: https://wiki.mozilla.org/Add-ons/Extension_Signing#Timeline

However, on Firefox for Android (at least up to 66.0.2), you can set xpinstall.signatures.required to false and bypass this problem.