Due to a large investment by IBM and the Mozilla community. Firefox is becoming as assessible as IE on Windows. The latest GW Micro private beta for Window-Eyes has support for Firefox, including support for the up-and-coming W3C accessibility effort to support scripted web content.
To find out more about Firefox accessibility check out the Accessible DHTML Preview. Aaron Leventhal is leading the Firefox enablement effort in Mozilla comunity.
Window-Eyes screen reader beta testers have indicated that they have faster access to the Web than IE.