Currently, I have used 2 web browser simultaneity. Internet Explorer 7 (IE7) and Mozilla Firefox 2.x.x (FF). Those two browsers did their jobs very well, and I'm still using them until this day. Despite many article war between these two by fanboys (I hate them..), IE7 and FF each one has its advantage and disadvantage.
Internet Explorer 7
I use IE on many ASP or ASP.NET site, which I know, Microsoft technology indeed has most compatibility each others. From my knowledge, IE7 has better support on CSS than Firefox, but in fact, I don't even bother since I don't really care about CSS stuffs. Many has said IE has lack of security, but for me, not this version. It has better security than previous one, embedded phishing filter (which I don't feel the difference if I don't use it), and one of the reasons, is a nice and better UI than FF. I usually use IE7 for navigate to Microsoft sites or development sites like Codeproject, Codeplex, etc. I also use this browser if I want to see a flash content of a site. I let windows update to manage updates for this browser.
Mozilla Firefox 2.x.x
Word to describe this broser: ADBlock Plus, NoScript and Load Images. I used ADBlock and NoScript recently and amazed about this plugin can do. Block those annoying advertisement or useless script about 90% of the time. Great plugin, especially when you're at a forum which has many advertisement content like Kaskus. For me, that site contain too much advertising and all of the are annoying. With this plug in, I've never encounter that problem again. The other future that I like is its abilities to block image only from selected sites. This help me a lot, rather than keep change settings when I browsing the net. It's lack support of activeX makes this browser save for some "dark" sites.
Browser war and fanboyism
With IE7 and FF currently dominate market share, it didn't surprised that there's a lot of debate to find out which one is the best. For me, neither two of them. Each browser has come with its advantages. It depends on how far a user will use this tools.