Installed plasma 6 this morning and noticed the same thing. Wasn't sure if it was ff or kde or some mysterious third option.
It does that when using X on my computer, but not when using Wayland.
I found that setting Firefox to fullscreen, then resizing it from there is a bit of a workaround that worked.
If you right-click on the Firefox toolbar, select "Customize Toolbar..." and then in the bottom left corner tick the "Title Bar" checkbox, does that change anything about it?
I only get this on my endeavorOS install when I use the ALHP x86-64-v3 repo as explained here:
https://somegit.dev/ALHP/ALHP.GO
When I get rid of them it went away so I've avoided them for a bit.
I believe this has been caught and fixed in the latest hotfixes. Have you got 6.0.1 available?
Yep, i'm on 6.0.1. Looks to be affecting Thunderbird as well.
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Plasma 6 Bugs
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