Good question, which I think I've answered though not in the way I expected to. (Didn't try switching to an HTML doctype, which I will try if I, ha ha, get a chance later).
No, I don't have a .browsers file, I dropped it and my App_Browsers folder when I dropped the CSS friendly adapters. I found them noble, but insufficient for my needs. The web server control architecture is fundamentally about one thing, eliminating the need to know XHTML. For a front end designer like me, who only came to server side programming after learning XHTML, then CSS, then JS, it's a solution looking for a problem. And given that I eliminate client miscommunications by switching from comps in the sketch phase to XHTML prototypes in the design phase, the architecture just got in my way. It was never designed to render a fine-tuned prototype and doesn't have the tools for that, so finally bit the bullet and just code my UIs using old-fashioned custom OOP techniques.
It is indeed firefox. There's some kind of JS plug-in, I suspect, doing the conversion. In order to more easily grab the code I want, I typically highlight around the area and choose View Selection Source. This opens a new window entitled, DOM source of selection and this is what I've been cutting and pasting above.
To do my test I did a simple View Page Source, so I could test the exact page, as is, as static XHTML saved with a .html extension. What I immediately discovered when I went to copy the code is that viewed this way in the "Source of Mypage.aspx" window does in fact show the "correct" version of these lines.
Based on that, since we can see the original code when looking at the source for the entire page, I believe that Firefox is getting the same XHTML as all the other browsers. After that, however, it's doing some kind of DOM manipulation. To me that probably means JS, which I believe is further strengthened by the dominance of JS as the language of most of the Mozilla plug-ins that I know. This doesn't make sense behavior wise for any of the plug-ins that I have, so I would guess that it's part of the core security of the browser. Especially since it's consistent on the old 2.0 version of one computer and the new 3.6 version I have on my other. It would be interesting to see if someone gets different results though using the View Selection Source option.