Here's the beginnings of another Firefox extension (it works, but not always, and not completely), designed as a minimal alternative to Songbird for when you just want to play a few tracks while you're browsing. It adds a media player and playlist to Firefox's sidebar and two entries to the contextual menu for links in web pages: "Play in Sidebar" (starts playing immediately) and "Add to Sidebar" (adds the track to the queue).
→Install XULVLC← (depends on the VLC plugin for Mozilla - mozilla-plugin-vlc in Ubuntu)
There's a bug at the moment where sometimes tracks don't get added, the VLC plugin is a bit crashy, and the playlist doesn't really do much yet.
The "Sidebar with size restrictions" userstyle could be useful for loading other audio bookmarks in the sidebar, but maybe it's easier to just keep them in a tab, or in Songbird.