<p>What are the main differences (if any) between the box models of IE8 and Firefox3?</p>

<p>Are they the same now?</p>

<p>What are the other main differences between these two browsers? Can a web developer assume that these two browsers as the same since they (seem to) support the latest web standards?</p>

The Internet Explorer box model has been "fixed" since Internet Explorer 6 so long as your pages are in standard compliants mode.
See:
Quirks mode
and
Internet Explorer box model bug
.
Until I learnt about doctype declerations getting IE to work properly was a real PAIN, because IE runs in "quirks mode" by default. So having a standards mode doctype will eliminate a whole bunch of the most painful CSS problems.