In BlueGriffon, I now have a wizard showing a panel when I click on a given button. That panel is by default semi-transparent because of its -moz-appearance. But semi-transparence is a real problem here because the panel contains a colour picker ! Of course -moz-appearance can be set to none but in that case, all other platform-specific settings of my panel are lost... Border, box-shadow, etc.

Seeing that Myk had the same problem and came to the same conclusion, I think that the power of -moz-appearance is unfortunately also a too strong constraint and the styles it applies should be overridable by !important declarations.