Enabling the browser plugin breaks the web interface of Emby on both Firefox and Chromium. See also https://emby.media/community/index.php?/topic/60046-strange-web-ui-problems/
I've been having trouble with the web interface of Home Assistant on Firefox lately, too. Turns out that too is due to plasma-browser-integration.
This is the web console output for Home Assistant: TypeError: this._meta.byKey is not a function[Learn More] iron-icon.js:181:12 TypeError: ServiceWorker script at https://home.mueslo.de/service_worker.js for scope https://home.mueslo.de/ encountered an error during installation. On Chromium it works fine with the plasma-browser-integration plugin enabled, though.
I've not seen any errors with Home Assistant. Can you give more details there on your setup? As for Emby can you confirm that disabling just the media player integration makes it work. Also please confirm your FF plugin version
I assume it's: 395531 getting me HTTP headers will help
*** This bug has been marked as a duplicate of bug 395214 ***
No, disabling just media controls does not fix it (none of the options do). I have to disable the plugin as a whole. As for the version of the plugin, in Firefox it just says "1.0", in the distro repositories it's 5.13.1-1.
Also, are you sure that the Emby error is due to iron-icons? It works neither in Chromium nor in Firefox.
Created attachment 113444 [details] Video playback page HTTP headers