I found a test page that will check your browser compliance with HTML 5 standard. I am using mostly Firefox at work and at home. I quickly run tests for 3 different browsers on my Mac and it seems like FF is not a champion of HTML 5.
1) Firefox 3.6.8 - scored 139/300 mostly because it has 0 or 1 in 6 categories: Parsing Rules,Local Devices, Elements, Forms, Microdata, WebGL
2) Safari 5.0.1 - scored 208/300. Still 5 categories were not covered.
3) Chrome 6.0.472.25 - is a winner with score 217/300. It looks like 4 categories are not yet implemented.
It seems like all browsers fully support Canvas, Geolocation and Workers. Safari and Chrome got almost 100% in Audio and Video categories, each one got one codec missing. Safari got best support for Forms and Web Applications but is totally behind in Files category.