How do I disable Firefox push notifications in Kubuntu?

The obvious solution is to simply disable or remove the Plasma Integration addon in Firefox. Without that, Firefox transfers are not integrated anymore into the Plasma notifications.

In case that integration is needed for some reason:

What is especially annoying about that pop-up is the fact that it stays open as long as the transfer. Thus it even covers the Firefox downloads button etc.

(What we don't want is losing completely all the transfer info by disabling only "Show in notifications" (as the initial answer told us to do): that would disable all progress, including Dolphin transfer which then cannot be monitored - as it takes place in the background, with just the task manager bar but no details and no possible intervention like stopping etc.)

That can be disabled by unchecking just the option "Keep popup open during transfer". After that, the popup will close after a number of seconds that can be reduced. Also, its location on the screen can be changed.

There is a way of removing Firefox transfers (and all transfers) from notifications without losing all info (popup or window) about that: by re-enabling the old-style normal transfer window as in old KDE and other desktops, by unchecking both task manager and notifications for "application progress"

Dolphin transfer then:

But that now includes the Firefox transfer too in a separate window (with Plasma integration addon), something some people may want

but it is embarrassing that for Firefox this kind of window is lacking the "Pause" button.

(The fact that we have to either disable only the "Keep popup during progress" -- and have the needed info in notifications for a short time that can be changed -- or both task manager and notifications which triggers a separate kwin window for transfers, but not just notifications is probably buggy, as that separate window should appear in all cases when the transfer is not shown in notifications.)