The configuration of the profile needs some extra modifications of the
browser profile:
The Javascript method dump must be active.
This method allows Javascrip code to write data to the output stream of the browser,
and can be activated with the config option browser.dom.window.dump.enabled.
Session recovery must be inactive
Session crash recovery will try to reopen the pages you were viewing before a browser crash,
but for this JOSM plugin, this feature is not useful. This feature can be deactivated
with the config option browser.sessionstore.resume_from_crash
These configuration properties can be modified automatically just clicking in the button below,
that will ask for permissions to do so. Just accept to allow the modifications.