Dare Obasanjo points out that it's not a sign of the apocalypse when an MS site doesn't support Firefox (or Safari, etc):

Unlike Devanshu and Todd, I don't think there are sinister conspiracy theories for why two Microsoft products were released and ignored features of interest to the geek demographic. In every product release, you have a limited amount of resources and time in which to apply those resources to your next version. This means that you tend to focus on features that will provide the most bang for the buck and may ignore features that have limited appeal such as supporting a browser which is used by 8% of the market or a media subscription model is only used by 1% of internet users. I don't always agree with the practice of deciding on features based on market penetration statistics but I can understand when product teams make such decisions. I suspect that is more likely the cause of these omissions than some nefarious collusion between MTV and the Windows Media team or some plot to ensure IE's market dominance by having Windows Live services require only that browser.

Of course, it depends on who your market is. For some products, ignoring that c8% might be mostly irrelevant, while for others, it's going to be a killer. On the other hand, not supporting them will be an ongoing irritant. I was trying to use some award miles on USAirways. I normally use Firefox, so I logged in and went to the award pages. I got all the way to the end, credit card entered, seats selected, the whole thing. Submitted, and bam - tossed to the reservation page with no confirmation and no warning.

Hmm. I verified that no miles had been taken out, and tried again. Same result. Spent some time on the phone with an agent who couldn't figure out how to make the reservation for me (this has to go into the annals of bad service. She was apologetic, but could do nothing for me). Finally, I tried IE. At the point of submission, it slaps up some "please wait" page that must be IE specific.

Now, for the guy trying to cash in award miles, that's irritating, but it's no lost revenue for the airline. Had I been trying to buy tickets though? I would have been far more worried about the "did that go through" thing had each one involved a potential charge of a few hundred bucks.

So I agree with Dare - resources and inertia explain these things far better than some silly conspiracy theory. Having said that, the one taking the damage from this sort of thing is the vendor with the badly implemented site.