This repository now used for GTK 2 builds, which all my older builds were. Now that GTK 3 is an option, those builds are in a separate repository.

https://build.opensuse.org/package/show/home:stevenpusser:palemoon-GTK3/palemoon

In any case, to add this repository and its apt key, click on the "download package" link on the upper right of this page, then:

Click on what version of Linux you are using is compatible with (as Linux Mint is compatible with various Ubuntu releases, and Mint's Debian Edition with Debian releases)

To add the repo and its key, click on "Add repository and install manually" link

Then cut and paste the directions for your particular distro's release into a terminal. Take care to use the directions for your release, since it's easy to mistakenly copy and paste the set above or below the correct ones--you can read what you've just pasted into the terminal to make sure it matches what you are running.

Note that if you add both repos, the GTK 3 version will be seen as an upgrade over the GTK 2.