Ok.. right off the bat I have some issues... Firefox seems to interpret Pixel dimensions and table settings differently than other browsers. Also.. there seems to be an issue with absolute positioning, which seems to be tied in with the pixel dimensions as well.
ActiveX doesn't seem to work at all, auto-popup blocking is nice.. but many sites use that as a functional part of the site.. although I'm sure that can be worked around.
Rowspan/colspan seem to work a bit differently.. something's just not quite the same.. can't quite put my finger on it atm though.
DHTML doesn't seem fully supported, motion is choppy.. as if graphics that are set to preload don't..
VBScript functionality is limited.
On the other hand, Built in Java Console and debugging scripts are nice, I'll definitely be using this to check my java coding.
Flash seems to have little or no issues, and I like the fact it playes in a separate window if linked directly to a .swf file.
XML Seems fully supported.
Meta tags are readily viewable under the view => Page information section, which I haven't been able to find an equivalent in Internet Explorer. I really like that as I tend to use meta tags to "sign" sites I create, as well as include keyword information and site descriptions.
Overall, for static or basic html pages, Firefox seems up to par. It includes a lot more functionality than I expected, and has some nice extras I rather like. For more dynamic, interactive pages, it seems limited in its ability to handle complex scripting at a reasonable speed.
I give it
(out of five)