I don’t know if it is related to your issues but here is what worked for me.
The resource manager was not working on my server. No directory listings, no displayed files, just a window with the resource type and an empty path across the top. It was the same in Firefox and IE.
In Firefox I was getting "this.DOMDocument has no properties" error in fckxml.js. After some digging I came across these pages
http://forum.mamboserver.com/showthread.php?t=89992 and
http://forum.mamboserver.com/showpost.php?p=397798&postcount=9 which suggested that I disable mod_security.c. After disabling mod_security.c in the connector/php folder the resource browser worked in IE but still not in Firefox.
I was about to give up but then I tried one more thing. I deleted my Firefox cookies (actually only the cookies for my site), closed Firefox, and tried again. Bam! Works. So here you go this may work for you.
1) Create this .htaccess file in \manager\media\browser\mcpuk\connectors\php to disable mod_security.c
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
2) Clear site cookies
3) Restart browser
I hope this helps.