On further investigating I found that showData called on mouse hover is fired with some issues in Firefox browser.
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echo "<td class=\"browsetd\" width=\"".strval(100/$imgsperline)."%\"><div style=\"$textStyle\" class=\"tdpic\"><a href=\"$SCRNAME?$GD_WORKAROUND$sep"."slides$sep".$i."\"><img border=\"0\" alt=\"".$filter_files[$i]."\" src=\"".$filter_files[$i]."\" onmouseover=\"showData($i,this)\" width=\"100%\"></a></div></td>";//was: width=\"$wwidth\" height=\"$wheight\"

showData