The "corrupt" message is a known issue and it is related to Firefox not trusting unsigned plugins which includes WebPlayer. This is detailed in http://www-01.ibm.com/support/docview.wss?uid=swg21975806&myns=swglotus&mynp=OCSSKTXQ&mync=E&cm_sp=swglotus-_-OCSSKTXQ-_-E
The same Technote suggests a work around which involves the user disabling this feature in FF. IBM are working with Mozilla on this. If you Google enough you can see bugzilla tickets from IBM raising tickets with Mozilla to sign previous versions of the plugin so I hope that it gets signed soon.
Also, with version 46 of FF users will not have the option to disable this check and the plugin will not work. Version 46 seems to be available for general release on the 19th April.
If IBM and Mozilla do not get this working then the plugin will not work but I know (IBM) they are working hard from discussions with their L2 guys in Dublin. Also, they are planning to do away with the need for a plugin altogether. That was on the cards about a year ago, I just hope it materialises soon as the plugin stupidity really gets in the way of being able to use the product.
I forgot to add that this problem will be fixed in Sametime 9.0.1 which will be released in the coming months but IBM will of course be fixing for the 9.0 code stream