1) That appears to focus on the DOCTYPE declaration which should be used for any web page, and tells the browser which set of standards that the page's creator wants it to be displayed in. I use the following DOCTYPE declaration:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
That's to declare that I used HTML4 coding standards which are still valid, and in theory to ignore any other coding tags, etc. Now that's a simplified statement, but that's what the DOCTYPE declaration is intended for, etc.
2) That doesn't appear to be relevant for my pages.
3) Yes, I've heard of that before.
4) As I said before, I think that I still have to associate your issue with how the new Firefox v4.0.1 interacts with my particular .cgi script and your bookmarked link (URL). The coding change that I made was really a 'workaround' that appears to have worked for your issue more than a change to bring my code into HTML5 specs, etc.
As for HTML tags that are depreciated in later W3C specs, unless specifically stated that they are being 'removed' for all previous specs and not to use them at all (which is rare), they are regarded as being 'backwards compatible'. If they were not backwards compatible, there would be a lot of the Internet that could not be displayed either partially, or completely in any browser.
In other words, if I declare that browsers should use HTML4, that's what they should do.
I'm sure that you, and others, have noticed that I've kept my pages somewhat 'basic' instead of using some of the bells & whistles that have been developed and used in pages just to keep up with other pages, etc. In other words, I want the focus to be on the questions and answers (content) instead of looking flashy just because I could do it.
Thanks for your time and efforts in looking into this!