The Addon tells me:

Verbindung zum nativen Host kann nicht hergestellt werden.

Stellen Sie sicher, dass das Paket „plasma-browser-integration“ korrekt installiert ist und Sie Plasma 5.13 oder später verwenden.

But got package plasma-browser-integration-6.7.3-1.fc44.x86_64 installed. Very confusing...

I finally found the app that caused a few pages not to load correctly. I never would have suspected Plasma Integration to be the culprit, but it turns I was wrong.
Try disabling it if you experience problems with webpages that load correctly in other browsers or private windows.
Otherwise it would actually be quite useful.

Полезная вещь!

Could be better if they released the fix to stop using InstallTrigger to detect Firefox, it's annoying to see that warning in devtools console all the time.

Nice idea but half baked. Doesn't show download speed, total file size, time remaining etc. All information easily available from within Firefox. Would be a nice addition to KDE but I will never understand when developers make decisions like this to do something nice but then completely half arse it.

Soundcloud cover doesn't show up, will rate 5 stars after next patch update regarding this.

Firefox is still opening files with wrong programs after installing this plugin. For example opening PDFs with Krita or GIMP instead of Okular. I just wanted Firefox to open files as configured in KDE-Plasma. I can manually configure Firefox to use the desired programs for each MIME time but there are several of them, and it does not even show Okular as an option so I have to manually introduce the executable path which in most cases changes between verions (IE a folder with the version number).

Understood is under development but the info are really short...

What is plasma-browser-integration?
Is it another plugin, a service, ....?

How can this be installed? Any link?
Thanks