This is not and has never been true. What is no longer possible is the ability to sideload extensions outside of the browser by copying files into a directory because that's how dirtbags have stuck extensions that could not be uninstalled into Firefox as part of a third-party app install.
Go to `about:addons`, install add-on from file, you're done.
Then I downloaded a working add-on. It worked fine when installing from file. I removed the signing information. It installs fine as a temporary add-on, but throws the same error as the hello world.
I'm pretty sure that the add-ons need to be signed by Mozilla to install from file.
https://extensionworkshop.com/documentation/publish/signing-...
You can then sideload unsigned add-ons by following these instructions:
https://extensionworkshop.com/documentation/publish/distribu...
And Google hasn't?