Oh, I didn't realize it wasn't available for Firefox Android. I use Kiwi on my phone and Firefox on desktop. A quick Google search says you can use Firefox Nightly to install custom add-on libraries, but I've never done that.
menu -> print -> "save as pdf"? seems to be there just like it's on desktop version
Turn on reader, then Menu > Print > select printer "print to PDF"
Ah that's nice. Thanks for the quick reply.