I see that for the issue: <Add-on> could not be installed because Firefox cannot modify the necessary file. The usual fixes include: - ensuring that xpinstall.enabled is set to true - ensuring that extensions.alwaysUnpack is set to true What might be the non-usual fixes as I have set both to true and I still get the message above. I have tried installing the <add-on> after I have set the values to true, then again after restarting my browser, then again after restarting my computer and finally tried creating a new profile and repeated each of the steps after setting the 2 preferences to true.