Also, Amazon Music (FKA Amazon Cloud Player) works with MP3 TenFourFox, but you still can't seek within the track (my bug), there's a quick snatch of music when switching tracks because of a race condition in the HTML5 audio stream (Amazon's bug), certain tracks are not properly detected as MP3 (Mozilla's bug), and if you skip to the next track if the current one is still streaming, it has to stream all the way before the next track will play (also Mozilla's bug). I used it to listen to Pink Floyd's new and allegedly final album The Endless River, which doesn't have much posthumous Richard Wright content after all and is overall weak and meandering even considering the general gestalt of the band's post-Waters incarnation and David Gilmour's avowed goal of an ambient album. Basically, it's a slower and even less adventurous On An Island, and it's kind of a letdown after the big pre-release buildup.
If these don't work for you, something's wrong, but it won't be fixed until the 38 betas. Sorry.