This page explains information that should be included when filing bugs related to Rhythmbox. Problems involving Firefox should be filed against rhythmbox.
Is it Gstreamer?
Note that Rhythmbox uses Gstreamer for decoding and encoding audio: if your problem is related to playback or encoding (transfer to a portable device), see if you can reproduce it in another Gstreamer-based application, such as Totem or Banshee. If you can, report the problem against gstreamer instead.
Rhythmbox can produce debugging output which may be helpful for tracking down behavior problems. Run "rhythmbox -d" from the command line and attempt to reproduce the problem. You can also turn on debugging for specific components, e.g. "rhythmbox -d podcast".
If you are experiencing problems importing a particular file into your Rhythmbox library, look at the Import Errors view (you can select it in the left pane). This will give details on the error.