the website says macintosh platforms are not supported.
If the website say's not supported, then the website is probably part of the problem as it's most likely not even attempting to send the video.
One thing to try is to lie to the website by changing Firefox's user agent string.
Enter "about:" (no quotes) in the URL text box and you'll see what Firefox reports - mine shows
Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.1.16) Gecko/2008070205 Firefox/2.0.0.16
The website parses that string, sees "Macintosh" and decides you're not worthy to receive the video.
Here's a list of user agent strings that Firefox reports.
To change yours do the following:
1) enter "about:config" (no quotes) in the URL text box
2) right-click in the list of preferences
3) select NEW->STRING
4) for preference name use: general.useragent.override
5) for value use one of the Window's strings from the above list, such as: Mozilla/5.0 (Windows; U; Windows NT 6.0; fr; rv:1.9.1b1) Gecko/20081007 Firefox/3.1b1
You'll know you did it correctly when the "about:" screen shows the new User Agent String.
If you want to revert to the original user string just right-click on "general.useragent.override" and select Reset. You can enter "useragent" in the Filter: box so you don't have to manually search the list.