How do you hide installed plugins in Firefox?
This website shows that too much about you can be seen: http://centralops.net/asp/co/BrowserMirror.vbs.asp
Most plugins don't need to be seen by content providers.
There isn't a way to do this yet in firefox. This is necessary so that the server you are connecting to can know what form to send the content back to you in. The closest analogy I can draw is how SSL sends all of the acceptable connection types (SSL1, TSL, etc.) and the server can pick what it wants to use to communicate. When we are sending all this information to the server we are letting it know that we have these plugins and it can send us content we will be able to use.