I would disable the plugin in the settings -> options -> extensions & add ons -> Plugins since it doesn't seem to matter if its working or not.

Thanks for the reply. I indeed tried to disable the plugin, widevine, but then firefox asked (in a yellow bar!) to allow DRM by enabling the plugin. Circular problem...

EDIT: widevine has nothing to do with the error message (the message still shows up when the DRM notification is hidden), but I did not find a way to disable "clearkey"

Some of those infobar controls can be hidden using custom style rules in a userChrome.css file. A good site to find fixes along those lines is the following subreddit:

https://www.reddit.com/r/FirefoxCSS/

notification[value="plugin-crashed"]{

display:none !important;

}

in userChrome.css solves the problem, thank you. However this workaround seems dangerous as it catches all crash notification.

notification[value="plugin-crashed"]{
 display:none !important;
}

in userChrome.css solves the problem, thank you. However this workaround seems dangerous as it catches all crash notification.

You might be able to limit the effect of the rule to full screen windows this way -- assembled from random web posts and not tested because I have no way of testing this:

#main-window[inFullscreen] notification[value="plugin-crashed"]{
 display:none !important;
}

#main-window[inFullscreen] notification[value="plugin-crashed"]{
 display:none !important;
}

Perfect! For information, I get this issue on this website: https://www.canalplus.com/, from the home page.