My system is Ubuntu LTS 20.04.3 with a KDE desktop environment and plasma-browser-integration. And my Firefox's got the latest Plasma Integration plugin. There are two things on Firefox that are capable of interacting with KDE's media players, one is the Plasma Integration plugin, another is Firefox own media controls, configured with media.hardwaremediakeys.enabled under about:config page, and is enabled by default. So I made a fresh installation, and left all Firefox's preferences to the default, the plugin provided a decent thumbnail of the on going video. However, when switching to private browsing mode, there was no thumbnail (permission for the plugin to work under private browsing mode is granted). And, after switching back from private browsing, thumbnails provided by the plugin is not coming back. However, after disabling the plugin, Firefox's will provide thumbnails with its own media controls. I'd purged Firefox and plasma-browser-integration, removed all Firefox profile by removing the folders ~/.mozilla/firefox/ and ~/.cache/mozilla/firefox/, and reinstall all the necessities, the bug can still be reproduced. Curiously, download notifications is somehow not showing up after all these steps.
I can confirm this behavior on Solus 1. Have the Media Player widget on the desktop, open my phone's notification (KDE Connect is configured to show the media player controls on the phone) 2. Load YouTube in a Firefox window using a profile with Plasma Integration as the only addon 3. Hover over a video until it plays, notice the Media Player widget and phone notification correctly show the pause icon, indicate the video is playing 4. Move the mouse so the video stops playing. Notice the Media Player widget and notification haven't changed. They still indicate the video is playing. Operating System: Solus 4.5 KDE Plasma Version: 6.1.2 KDE Frameworks Version: 6.3.0 Qt Version: 6.7.2
The last comment was meant for another bug. To clarify my testing: I am able to reproduce the problem with the Media Player widget, but I cannot reproduce the second problem with going back to the non-private browser window 1. Have the Media Player widget on the desktop 2. Load YouTube in a Firefox window using a profile with Plasma Integration as the only addon - at this point, previews are working normally in the browser and widget 3. Open a private browsing window with the same profile (Plasma Integration is allowed) - there is no tab for this in the widget as there should be 4. Go back to the non-private window. The widget is still behaving normally. 5. Close the private window, interact with the thumbnails in the main window, the widget is still acting normally Operating System: Solus 4.5 KDE Plasma Version: 6.1.2 KDE Frameworks Version: 6.3.0 Qt Version: 6.7.2