They were using WMV early on. It was really easy to find ripped versions of episodes elsewhere online. They switched over to Flash but it required some sort of ActiveX to run it. (They offered plug-in to run it in FireFox on Windows.)
They had a single WMV episode open whenever you tried to load this version as a small consolation to Mac users. I noticed recently that it switched over to a new Flash player for OS X. Now I guess that player is used full time for the Fix viewing on OS X. I had several of my friends send in e-mails complaining about the lack of non-Windows support.