imanager plugin stops FCK loading in Firefox

FCK loaded fine in Firefox until I added the imanager plugin. Now, it works fine in IE, but in Firefox I get repeated messages in the javascript console:
Error: Unknown property 'filter'. Declaration dropped.
Source File: http://.../admin/FCKeditor/editor/skins ... editor.css
Line: 113

Error: Error in parsing value for property 'cursor'. Declaration dropped.
Source File: http://.../admin/FCKeditor/editor/skins ... xtmenu.css
Line: 42

Error: Unknown property 'filter'. Declaration dropped.
Source File: http://.../admin/FCKeditor/editor/skins ... xtmenu.css
Line: 53

Error: Unknown property 'text-overflow'. Declaration dropped.
Source File: http://.../admin/FCKeditor/editor/skins ... xtmenu.css
Line: 172

Here is my initialization code from inside my PHP script:

<?php
$oFCKeditor->BasePath = 'FCKeditor/editor/fckeditor.html' ;
$oFCKeditor = new FCKeditor('FCKeditor1') ;
$oFCKeditor->Value = $final_content;
$oFCKeditor->Width = "80%";
$oFCKeditor->Height = 400;
$oFCKeditor->Create() ;
?>