Please head over to the website http://www.fossamail.org to get it
Please note: this is a first release for this platform and there may be some bugs or quirks - it's not 100% clear which kernels/distros this will run properly on, but it should be good for any semi-recent Linux distro and beyond.
EDIT: For those who may have trouble with fosshub (you never know), here's an ftp alternative -- includes PGP signatures as well.
ftp://fmlinux:get@ftp.palemoon.org/
Note that there are .gz versions of the .xpi for convenience (so you can just click those instead of Alt-clicking or right-clicking the raw .xpis to prevent Pale Moon from trying to install them
)