* Safari is just as fast as Chrome now, as far as I can tell. The main competition between them is the javascript engine. They use the same rendering technology (WebKit), which is much better than Firefox's Gecko. Everyone is abandoning Gecko. For example, Gnome's Epiphany is now WebKit-based. Whatever the best browser is, it's going to use WebKit.
* Because of its non-native UI, Firefox is ugly and behaves differently than other applications. Because it is based on Gecko, it is slow and not as standards-compliant as the competition. I keep it around for weird extensions (Download Them All, etc), but I don't need it for day-to-day browsing. The only extensions I need/use every day are 1Password, XMarks, Click to flash, and Ad Block, all on Safari. (1Password, by the way, is the best software I purchased in 2009. I ignored numerous recommendations for it for a really long time.)
* It is possible to have more than one application installed on a single computer.
* These aren't browser specific, but I use the Printliminator, Readability, and Read Later/Instapaper bookmarklets every day.