MicrosoftCorporation's WindowsMedia formats include .ASF (colloq. "asshole saggy format" or perhaps active streaming format for those less ass obsessed) .WMA (Windows Media Audio) and .WMV (Windows Media Video). Another closed-source codec; similar in scope to RealPlayer.
If you install Microsoft's codecs (w32codecs for Debian), you shouldn't have trouble playing these on Linux with MPlayer. To get MozillaFirefox support going you might need to install Xine