Several rendering issues in FireFox 3, page doesn't load fully in IE7 (probably the two most popular browsers and it's not working correctly in either - this needs some fixing).
Other than those, the site looks good, but it shouldn't be having serious rendering issues.
The probable cause?
http://validator.w3.org/check?uri=http%3A%2F%2Fwww.richmixradio.com%2F&charset=(detect+automatically)&doctype=Inline&group=0
Errors found while checking this document as HTML 4.01 Transitional!
Result: 93 Errors, 154 warning(s)
Since FF and other browsers try to render by the standards, having 93 errors is not really a good thing. One is an easy fix though - your Favicon <link> needs to be BELOW the doctype. Doctype must be the very first thing returned in an html document for it to know what to use, otherwise it defaults to 4.01 transitional (the doctype you've got is for xhtml 1.0 transitional, but since it's second, it starts automatically parsing as 4.01).
Also, I'm not entirely sure if it's my pc or your site, but roughly half the page fails to load -at all- in IE7 (see screencaps below)
I gotta be honest though, fixing the doctype is the least of the problems.
http://validator.w3.org/check?uri=http%3A%2F%2Fwww.richmixradio.com%2F&charset=(detect+automatically)&doctype=XHTML+1.0+Transitional&group=0
When forcing validation as xhtml1.0 transitional:
Errors found while checking this document as XHTML 1.0 Transitional!
Result: 141 Errors, 2 warning(s)
1 of those is the doctype after the link tag though.
You've got some work ahead of you; the site looks like it's got the makings to be awesome, but it needs some tweakin' so it actually loads all the way in IE, and so everything's back to where it should be in Firefox.
IE7 Failed Load:
www.livewire90210.com/down_files/richmx%20failed%20load.jpg
FF3 Poor Positioning:
www.livewire90210.com/down_files/richmx%20firefox3.jpg
FF3, from the top-left: Blue hyperlink is almost invisible, and even without the jpg quality loss it's still barely there, the text is outside of the gradient areas for recent posts/topics, article author/category blocked by image, the two arrows by "wall" are actually covering part of the word, calendar isn't centered.
Sorry if this seems harsh, but if everyone viewing the site in IE7 gets presented with a site that doesn't fully load...Well, given that the IE series is still the most commonly used browser, most of your viewers aren't going to be able to see the site. As for the errors in Firefox, I've seen other Wordpress sites that render without errors/warnings, so it's not a Wordpress problem - it's a theme problem.