This is directly after a fresh install of KDE Neon Unstable and running an update. I'm not sure if this was the case before updating. Upon opening Firefox there is no visible close button, but you can still click on it and the highlight affect shows when you hover your mouse in the correct location, but still no X. To make sure this didn't have anything to do with my extensions (I didn't notice the issue until after logging in to my Firefox account), I opened a private mode window and the problem persisted.

The only thing special I did during the install was manually partition to a very standard BTRFS layout.

Changing the theme in Firefox fixes the issue, so it must be a problem with the default Breeze theme.


STEPS TO REPRODUCE
1. Install KDE Neon Unstable
2. Update to KDE Plasma 6.0 Beta 1
3. Open Firefox
4. Cry a single tear

OBSERVED RESULT

No visible close button.

EXPECTED RESULT

Visible close button.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon 6.0 Beta 1
KDE Plasma Version: 5.90.0
KDE Frameworks Version: 5.245.0
Qt Version: 6.6.0
Kernel: 6.2.0-37-generic (64-bit)
Graphics Platform: Wayland
CPU: 16 × AMD Ryzen 7 3700X 8-Core Processor
GPU: AMD Radeon RX 5700 XT
15.5 GiB RAM