About my system:

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Operating System: Kubuntu 22.04
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.3
Kernel Version: 5.15.0-40-generic (64-bit)
Graphics Platform: X11


I also had this issue on older Kubuntu.

From some reason sometimes apps in KDE Menu get broken. Like on the screen below:

It's also happening on the desktop panel in the same time. I'm not sure what is causing this.
It's not only disappeared icon. It also lost menu to add it to panel/desktop etc. When I do right click on this I have only Run in Terminal Window. it's not only related to Firefox.

Hello,
While I don't know why this happens, a quick fix could be just adding Firefox back to the menu using the “Menu Editor”. You can right-click the Application Launcher, choose “Edit Applications” and create a new item under internet category. Then just put Firefox (with a capital F) in the name box, and firefox (with a small f) in the command box, and select the Firefox icon as well before saving. That should put Firefox back in the applications launcher.
Hope that helps