No, this uses KDE Connect, kdeconnect-handler is a normal part of it. But Plasma is the issue. There is a Plasma Integration for Firefox, which seems to be capable of sending URLs via KDE Connect.

If I want to send a URL to my TV I’d otherwise need to copy the the current URL from the location bar or "Copy Link". Open kdeconnect-handler, select the device, select "send URL", paste the URL and klick "OK".

kdeconnect-app (a window; opposed to the popup dialogue of handler) and kdeconnect-indicator (tray icon) don’t support sending URLs.

With this addon I can right click on a tab or link, select "KDEConnect", then just select the device and hit OK.

kdeconnect-app and kdeconnect-indicator can send the clipboard, but that’s a different feature. My FireTV can run KDE Connect for Android, but it requires a physical keyboard or a third party softkeyboard to paste from clipboard. Also not all apps support pasting URLs. A sent URL in contrast will be opened with apps registered for the domain.

It’s a bad example for this, but if I pasted https://f-droid.org/de/packages/org.kde.ktrip/ in the search of F-Droid it doesn’t know what to do with it. If I send the URL F-Droid opens the package description. The search would only work for org.kde.ktrip or ktrip.