Hi Steve & Madelaine...
I had the same problem with the realplayer plugin. I downloaded the 'YouTube Downloader', but it wasn't anywhere as near user friendly.
My workaround was to download
Nightly Tester Tools , a Mozilla addon.
This is a tool that lets developers work with Mozzy. Once installed, restart firefox.
Then go to tools>>addons....
You will see a box in the bottom right hand corner that says 'override all compatibility'
It will warn you that forcing addons to be compatible can mess your FFX up, so do this at your own peril
It then goes through your incompatible addons 1 by 1. I just skipped the ones I didn't want.
My RealPlayer Browser plug in now works, I get the popup at the top if the youtube vid, and it downloads straight into my realplayer library.
I don't seem to have encountered any problems with FFX since doing this.
Hope this helps you,
Mugsy.