palemoon and the precompiled palemoon-bin are both still gtk2-based versions, but gtk2 has already long been dropped from the Arch and Manjaro repositories.
I would recommend trying the gtk3 variant, of which the precompiled version is palemoon-gtk3-bin. 
pamac build palemoon-gtk3-bin
I would not recommend building it from source code — i.e. palemoon-gtk3 without the -bin — unless you’ve got a couple of hours to spare. Six years ago, I built a firefox from sources, and my computer was completely locking me out for over five hours straight, with all cores at 100% and all of my RAM in use.
Better go with the -bin. 