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Personally, I'm not a big fan of either IE or Firefox, nor of Opera. Please note that the following are my opinions, and should not be read as if facts.
Internet Explorer: I like it, whenever I use Windows at university, because of its speed. It loads pages fast, the program and new windows load fast. I find that a huge advantage over any other browser out there. The fact that this speed might result from the fact that IE is tied with the underlying OS is irrelevant. Speed is speed.
IEs downsides to me lay in the security area. Even though spyware, viruses and attacks have never posed a problem to me at all (the only virus I ever had in 15 years of computing was the Junkie boot virus-- removed command.com ), I do see less experienced people around me having problems with it.
Firefox: I use it daily on my main machine (Firefox 1.0.6 on Ubuntu Hoary), however I'm not fond of FF because it simply feels a lot slower than other browsers, both in program/window loading as in page loading. Please note the use of the word "feel". I have had this problem with browsers based on the gecko engine for a long time, but I in no way can exactly pinpoint the slowness. It's a very subjective matter, and a lot of people will rightfully so disagree with me.
I use Firefox on my Linux box because it simply is the best browser on Linux, period. On Windows, I'd use both IE and FF. However, I don't use Wndows.
Opera: I don't like Opera because I find it bloated. To me, an application is bloated when it has too many features and things I don't need. Opera is such a piece of software to me.
Then, which browser do I prefer? Safari, hands down. Safari 2.0 was a setback over the previous version that came with Panther. I don't see the use in RSS personally, so Safari 2.0's RSS feature is bloat to me. Secondly, Safari 2.0 has, like all of Tiger, problems with screen remnants; especially in text input fields (lines double printed) and scrollbars (the scrollblob being copied too many times). And above all: you cannot remove that damn Google search field in Safari 2.0. I *hate* that completely *useless* thing.
However, all in all, Safari is fast, provides the best CSS2 support (try rendering my blog, http://cogscanthink.blogsome.com, in Safari and FF/IE; only in Safari does the header render properly (with shadow)), and all in all looks the best too.