KDE Neon unstable, cursor small on FireFox. FireFox close button invisible

I just installed Neon and updated it all, to test KDE 6.0. I have not changed settings much other than changing the theme to dark and increasing the cursor size in the settings. It seems that the cursor becomes really small on non QT6 apps like FireFox or MPV. Also, FF’s window buttons are empty. I am using Wayland, 125% scaling.

Well, I tried to upload the screen recording that was recorded by the software that was installed by default, but this forum rejects the file format WEBM. This seems stupid. The file size is only 15KB. Why is GIF, which can be larger than 15KB allowed, but not the screen recording WEBM? Converting WEBM to GIF would make the file larger and converting itself is cumbersome. I had to upload it to imgur.