I can confirm this on Ubuntu 19.04 with Firefox 66.0.3. Changing xpinstall.signatures.required from the default 'True' to 'False' resulted in addons working again.
Android too.
Worked for me on Linux, didn't even require a restart (some addons just reappeared immediately). Using Firefox from Ubuntu package management.
AFAIK that only works for the nighty/dev versions.
but only for dev/beta/nightly. Note that this won't work for the standard version of FF.
Curious, that shouldn't work on branded stable installs.
It works on Linux but not Windows or Mac installs. I assume that repo maintainers use different compiler options than the official Firefox binaries.
For me this is False ;-)
Are you using an Extended Support Release (ESR)? That's expected then.