This extension allows you to queue up songs from the library into a collapsible pane on the right, and then play them as a playlist. You can also save the list as a playlist if you like.
I have re-enabled preferences, please let me know how they go. The preference page is unlocalised at the moment, I will be posting the new version on Babelzilla shortly.
Also this version shouldn't start downloading all the tracks if you play a web playlist.
Source code is available from a mercurial repository.
hg clone http://hg.mozdev.org/nowplayinglist/
* updated Spanish (es-ES) and Traditional Chinese (zh-TW) locales