I recently discovered there is a "plasma browser integration" extension for Firefox. I installed it but I have no idea how to access it. Here is some info from the KDE wiki.

Media Controls

  • Lets you control video and audio players in websites using the Media Controller plasmoid, media keys on your keyboard, and even remotely from your phone
I don't know what the Media Controller Plasmoid is. The only media keys on my keyboard I know of control the volume up and down.

Show downloads in and control them from Plasma’s notification area
I don't know what the Plasma notification area is.
Make sure the "Browser Tabs" module is enabled in Plasma Search (KRunner) settings
I'm not sure what this is either. This extension sounded like something I might find useful but I'm totally confused.