It's gone for me too, at least from the web player. This is a very important feature that I use every day and has just gone missing.
I like the look of the new update, just it's removed heaps of functionality that I used to use. My keyboard shortcuts no longer work, I used to be able to pause, play, skip ect from anywhere within firefox but now all gone!
What exactly I dont see is the button that allowed me to select a song and add it to play next. Capturing a screenshot will show you that it's not there. I will reinstall the app as advised. Thank you.