I run XP and use both Opera and Firefox.
Opera is the fastest browser.
There are things I like and dislike about both browsers.
Since this post is about opera I will talk about Opera.
When I was first introduced to Opera it wasn't freeware.
I bought a student license and was up and running.
From a security perspective the developers of Opera are
serious about patching any vulnerabilities with there product.
For a reference look @ this:
Vulnerability Report: Opera 9.x
Note: The green check mark means the issue has been fixed.
Vulnerability Report: Mozilla Firefox 2.0.x
Firefox makes up for this with there extensions/add-ons initiative.
Opera has many features pre-installed:
Opera Security Features
Web Specifications Supported in Opera 9
From a web developer perspective I place this
browser on top of the others. When coding
and testing on other browsers something that
makes logical sense coding wise does not respond
well in other browsers. Firefox rarely has issues.
Its been awhile since working on a web project so
I can't speak for IE7.