I prefer Opera - have you tried the latest official version 9.20 ?
If a site doesn't recognize it (rare these days) I just choose the option to Identify as Firefox.
I use v9.21 build 8776 which I find stable, from the latest build of Opera : http://my.opera.com/desktopteam/blog .
I find it has so many native features I don't look for add-ons.
From what I hear, IE7 still has lots of bugs, incorrect implementations, incomplete CSS2.1/DOM2/HTML4 support, incomplete or buggy support for CSS 2.1, HTML 4 and for DOM 2.
Opera supports all major Web standards currently in use, including CSS 2.1, XHTML 1.1, HTML 4.01, WML 2.0, ECMAScript, DOM 2 and SVG 1.1 basic.