Here's what I found. First, it's always good to run a page through the w3c validator to check for errors. Usually rendering problems are caused by a code error. In this case there are lot.

That said, you are right that it works in IE and Chrome. Different browsers deal with errors differently. From looking at the source code and error messages, my guess is that is has something to do with the <font> tags. The first one is opened on line 61 and don't get closed until line 821. In between, they are all nested one inside the other. Ultimately, that may not be the cause but it's hard to tell until things like that are fixed.

That is a problem with the HTML5 parser and the above mentioned problem with the unclosed FONT tags. That is causing a lot of deep nesting and the HTML5 parser can't cope with that. In older Firefox versions it was possible to disable the HTML5 parser in such a case as a work around, but that is no longer possible. So that code needs to be cleaned up.

• bug 354161 - DOM pruned at depth 200

(please do not comment in bug reports)

Thank you. Will get in touch with the site owners and tell them. When I learn the result, will get back to the forum.

Suggested to site owners they look at your explanation of the problem. They've done so, applied the solution suggested, and ... problem solved! Thanks all round.