I noticed it just after I upgraded to Firefox 22.214.171.124 on Windows XP.
The cause, for me, turned out to be the "RealPlayer Browser Record Plugin", which had appeared as part of the lastest upgrade to Real Audio's RealPlayer
I saw recommendations to uninstall this, but I found that disabling was sufficient.
Don't ask how long I spent faffing about to track that down...
- 25 Feb 2008