Minor Theme gzip compression was breaking CSS in Firefox.
It seems that no Content-Encoding header is being sent to the browser.
Following change to function send_cached_css() in /theme/styles.php sends header, resulting in CSS being applied to pages correctly. Similar change should probably be made to /theme/yui_combo.php as that also sends garbage to the browser.
if (!min_enable_zlib_compression())
{ header('Content-Length: '.filesize($csspath)); }else
{ header('Content-Encoding: gzip'); }