I am wondering whether this is due to the login page using proprietary MS fonts. I had this problem with trying to access the settings page on a modem/router. I could access the page with IE, but could not see much of the content in Firefox.
In Firefox you could try the Edit -> Preferences -> Content -> Fonts and colours [Advanced] button to set Fonts to Western and Character Encoding to Western (ISO-8859-1).

What they mean by IE-only supported is the webpages they have are probably heavily using ActiveX. I've used FireFox + Linux on a Windows+IE only network just fine.