In the thread, multiple people answered his question. Here are the two most significant reasons that I noticed.
1. The prefix was already not being supported by most browsers, thus supporting it would mean supporting the development of websites that don't work properly in other browsers.
2. Best practices suggest that one shouldn't rely solely on the prefixed version of an experimental alias. If this developer had written good code to start with, he wouldn't have had the problems he ran into.
The Mozilla developers explain the reasoning at length.
They don't want to tie people to Firefox. So when a standards-compliant version of a CSS directive becomes available, they deprecate their own -moz version.
Eventually they remove it.
The goal statement is that a page written using Firefox as the development browser will look the same for Chrome, IE and so on.
-moz defeats that purpose because it's browser-specific.
