Can't install SDK add-on

When trying to install my add-on (locally from https://github.com/brettz9/htmleditor-addon
or from https://addons.mozilla.org/en-US/firefox/addon/htmleditor/versions/ ), I get the message “HTMLEditor could not be installed because Firefox cannot modify the needed file”. What could be the source of this problem?

It was packaged via “jpm xpi”

Worked fine for me.

It could be a problem in the local file system, like having the profile directory be read-only, or some other issue. I remember seeing this error before, but I’m not sure what the cause was then.