I know this is possible, but first you actually have to install IE on your linux machine via WINE and whatnot. It get's a little messy, and is kind of slow.
I personally prefer konqueror to firefox, btw.
If you happen to use it, the relevant settings are under Settings->Configure konqueror->Browser Identification.
You can get an ubuntu package of Opera from them here.
When you've saved it you should be able to "dpkg -i opera.something-9.02.deb" to install it. (Or look for some way of installing packages in synaptic, or you can just try doubleclicking it and seeing what happens.)