I have the same issue as the thread below. Middle mouse always pastes no matter what.
By default Manjaro with KDE disables middle mouse. Instead it always pastes. Does not matter what programs you are in. This is what the OP was asking about. OP in that thread can’t use middle mouse to scroll on Firefox, or to shift around the view in Blender.
I’ve tried Right Click on Desktop → Configure Desktop and Wallpaper… → Mouse Actions and changed from Paste to Standard Menu, or simply deleting it altogether.
Unfortunately the Paste function is still the only thing that happens on middle mouse press.
Using KDE search function does not pop up anything other than System Settings → Mouse which only has speed, invert scrolling etc. Nothing about mouse buttons.
Logging out makes no difference. Standard Menu works immediately for the desktop itself only (not even Dolphin, the file explorer). If I open Firefox and press middle mouse it still pastes whatever is on clipboard instead of doing the normal scrolling up/down arrow pane.
It seems Firefox just defaults to not having this functionality, but it still exists.
It can be enabled with:
about:config → middlemouse.paste → false
Settings → Browsing → Use autoscrolling (enable it)
So for a lot of programs the feature still exists, just need to find the individual program tweaks.