Strange... I just went to the Mozilla/Firefox sites, and there's no mention of 3.5.3. I tried "check for updates" within Firefox, and it also said that my 3.5.2 version was current.
Are you subscribed to the Firefox 'beta' release channel?
More on-topic, it's not clear whether the problem reported above has to do with Firefox's 'reporting' of the installed .NET version in the User-Agent header string that it sends its with its HTTP requests, with Firefox's 'hook' to pass .NET requests to the OS, or with some flaw in .NET preventing it from working with the new Firefox release (presumably due to a .NET interface documentation error). The reports seem to be lacking in facts and technical details.
I haven't seen a problem with .NET and Firefox/3.5, but maybe that's because I haven't visited any sites that require .NET. And I suppose that if I did encounter a problem, I could just re-visit that site using IE8, just as I do with the Microsoft Update and Windows Update sites that require IE... I *use* browsers, but I'm not "devoted" to any one of them. As such, I guess don't see what all the excitement is about.