I'm finding more sites like this AMEX site. Firefox worked well on the American Express site until they redesigned it, now it doesn't work (I'm using 1.5b2, maybe it still works with the release version?). I'm seeing that more often now. I have a sneaking suspicion the new generation of Microsoft's web tools break non-IE browsers. Shocked, shocked I am.
This might explain why Firefox use is declining.
There's no place on the AMEX site to complain of course.
[Update 10/11: My caveat about 'maybe it's the beta' may have needed more emphasis. My sister-in-law reports the site still works with the release version of Firefox. I've submitted a report to Firefox on this site.]