I use firefox 59 (nightly release) on Arch with the KDE Plasma desktop. I moved to the nightly release to hide the titlebar and use client side decorations. I've hidden the titlebar. My tabs are now adjacent to the top screen edge.

I can't select a tab when I move my mouse to the top screen edge. I activate a plasma window move function instead. The Firefox window goes translucent, the mouse cursor changes to a four arrow cursor. I have to moved the cursor to just before the screen edge to successfully select a tab.

In case someone is wondering, I enabled widget.allow-client-side-decoration in about:config to get rid of the title bar.

Is there anyway to select browser tabs when I move my mouse cursor to the top screen edge, instead of activating the window move function?

I think my issue is an issue with Firefox, not plasma. Bug report below. There was a fix, but it apparently stopped working with the release of Firefox 57.

https://bugzilla.mozilla.org/show_bug.cgi?id=1224206