Definitely Firefox for most of my browsing, with Chrome for SWMBO and quick browsing when necessary. HTML5 for youtube etc under Chrome is horrible - fails far too often, so I usually don't bother with it for that.
Ads annoy me and as mentioned above, only FF cuts them accurately and completely. It is a CPU hog, but killing it every couple of hours, if you haven't already closed it for some other reason, does the job just fine.
[update]
Looks like Chrome is getting there with the plugins, especially (as mentioned above) the Lifehacker ones. My wife's chrome instance ain't too bad now.
Edited by frog51 (23/02/2010 11:28)
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Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock