FireFox is compatible with the HTML 4.0 & XHTML 1.1 Internet Standards.
.. it's just a shame the internet isn't
If you follow the Internet Browser sites about the vunerablities, you'll have noticed in the last year there have been more problems found with Mozilla (and some specific to FireFox) than were found with Internet Explorer.
We're talking bugs and vunerabilities, not compliancy to standards.
One comment on one of the anaylst sites summed it up quite well,
"Explorer know to be fully of more holes that cheese that the swiss make, but at least it lets you know when it's being attacked. Even if it isn't going to do anything about it."
It takes Microsoft longer to fix bugs, but they do produce more bug fixes every week than any other developer. They also have been correcting bugs for years, not months.
Really it's just a matter of 'there are only so many bugs', so it's just through process of elimination really that we're getting something more stable to work with.
Similar with Dark Basic Professional really... we all know it has it flaws but each patch does get us closer to that ever illusive stable patch.