Plasma Browser Integration is a suite of new features which make Firefox, Chrome and Chromium-based browsers work with your desktop. Downloads are now displayed in the Plasma notification popup just as when transferring files with Dolphin. The Media Controls Plasmoid can mute and skip videos and music playing from within the browser. You can send a link to your phone with KDE Connect. Browser tabs can be opened directly using KRunner via the Alt+Space keyboard shortcut. To enable Plasma Browser Integration, add the relevant plugin from the addon store of your favorite browser.
Before installing the browser's extension, you need to install the package `plasma-browser-integration` (the package name may change between the Linux distribution).
Now you can install the browser extension like a normal browser extension.