I’m having an issue in both firefox and thunderbird. Dialogs do not work well. I think it should be something related to GTK, so I’m trying to change GTK settings. I remember GTK setting to be in “system settings” in plasma but I can’t find them. I’m using opensuse 15.2 with plasma 15.18.5.
Maybe you are looking for System Settings > Application Style > Configure Gnome/GTK Application Style (a button near the lower side of the settings window)