I intend to land bug 1620467 tomorrow, which implements the unprefixed appearance property and removes many non-standard values of -moz-appearance.
Intent to Prototype mail: https://groups.google.com/g/mozilla.dev.platform/c/nlun5QV63Bo/m/xUXCmySGAgAJ
The only things that have changed since that mail of a couple of weeks ago are:
* Chrome M84, which equivalent changes, has now reached Beta.
* Chrome goes one step further and makes `appearance: button` be treated like `auto`, rather than having an effect on all elements except a few. The telemetry I gathered for this showed a low number of pages would be affected by this. I plan to make this change only once the corresponding Chrome change has made it to release and has been proven to not need backing out. This is tracked in https://github.com/w3c/csswg-drafts/issues/5174