Opera has identity issues
There's a small number of differences between IE and FireFox that I resolve by detecting the browser. As near as I can figure out, Opera pretends to be both browsers, probably because its users complained that they'd get locked out of sites that require a standards-compliant browser. Problem is, it's not standards-compliant, even though it falsely claims to be a standards-compliant browser, and screws the pooch pretending to be either IE or FireFox.