Ok - did some more investigation and it now seems it's quite a specific issue on Firefox (3.0.14 AND 3.5.3 both) on my MacOSX Macbook.
Safari on the same machine does not have this issue (has some other issues btw with the admin side of things).
Firefox 3.5.3 on my Windows XP testmachine is working fine..
Very strange - but for now, I'm assuming it's a local environment thingie and maybe not a specific SS issue.
Thanks for your help so far.