Amazing add-on. I love it. Can't live without it. This review might be a bit long, but I really love this plugin and want to help you. Keep up the great work you're doing now.
There is a couple of things I would point out.

FIREFOX FEATURE:
Can you add option "Open on phone" when I right click anywhere on page (right now it works only on links).
Now I need to click share and go through multiple popups and clicks. And when I share page like this, plugin crashes when I exit firefox (nothing happens, I just get notification). There is no crashing when I right click on link and share.

BRAVE FEATURE
Not sure if you also manage brave plugin but I will say it here.
In Brave browser when I interact with media controls on my phone, notification loses the cover art from video (example: youtube) and becomes orange. When I play videos in firefox that doesn't happen.

Kills all thumbnails on all browser windows by showing the video thumbnail and player controls - even on windows that have a different web page loaded, or even after you went away from the video onto another page. It can also not handle multiple tabs / windows with different videos.
In its current form a clear downgrade to the regular functionality of previewing the current webpage of the window.

Great addon!

There is one main issue I would like to point out: If I remap the keyboard keys in KDE System Settings > Global Shortcuts, I am unable to use the remapped keys in Firefox and some other programs. I am not sure if it is possible for the extension to make it possible for that to happen, though.

Works after copying the native messaging host (org.kde.plasma.browser_integration.json) to ~/.mozilla/native-messaging-hosts

This is simply a superb addition to one of the best desktop enviroments going