If it's a legacy browser, it shouldn't have X/HTML delivered to it - I suspect what cLive ;-) is doing is delivering X/HTML as HTML, which would cause Mozilla to go into tagsoup mode (i.e., you're not in strict or even transitional, you're in nothing).

I tested my Moz, and <br/> is equivilent to <br></br> in an X/HTML document delivered as such. For HTML documents, <br> is the only valid markup - <br/> won't work, and <br /> is invalid HTML (although they do suggest it in the XHTML spec as some kind of backward compatible thing, which is wrong IMO).

Such is life :( I tend to deliver valid X/HTML to those clients that can handle it, and tagsoup to the rest.