Very nice design The green bar and tabs don't blend in too well.. maybe add in a dropshadow of somesort there for a smoother transition?
I think the light blue background on the SourceCrave Tutorial headers needs to be darker/stand out more. Hard to differentiate between header and description. Also your navigation links on the left do bad things onmouseover in IE 6. I think with a few small changes it could look very good!
to the right of the upper logo, there is button for #1, I think it looks kinda of out of place, maybe it's the rounded edged black box around it... I love the colours. i would add a little space between the username field to the home tab.
urm, do you honestly doubt that? Check your webserver logs for proof. What do you suggest the most common browser is? FF? lol