How to add specific library dependencies for a Snap?

I’m trying to build Pale Moon as a Snap (never did it before, so I figured I’d start with one of my favorite alternatives), and it returned this:

$ snapcraft          
Using 'snapcraft.yaml': Project assets will be searched for from the 'snap' directory.
Launching a VM.
2019-06-30T21:51:24Z INFO Waiting for restart...
core 16-2.39.3 from Canonical✓ installed
snapcraft 3.6 from Canonical✓ installed
core18 20190611 from Canonical✓ installed
Using 'snapcraft.yaml': Project assets will be searched for from the 'snap' directory.
Pulling palemoon 
Downloading 'palemoon-28.5.2.linux-x86_64.tar.bz2'[=================================================================================================================================================================================================================================================================================================================] 100%
Building palemoon 
Staging palemoon 
Priming palemoon 
The 'palemoon' part needs the following libraries that are not included in the snap or base: 
These dependencies can be satisfied via more stage-packages, more parts, or content sharing.
Snapping 'palemoon' \                                                                                                                                                         
Snapped palemoon_28.5.2_amd64.snap

Aren’t Freetype and PNG libs in Ubuntu 18.04?

In the parts section, specify them in a YAML array, like
build-packages: [libglib2.0-dev,libpulse-dev,libfdt-dev,libpixman-1-dev,zlib1g-dev,python3,libspice-protocol-dev,libspice-server-dev]
stage-packages: [libglib2.0-0,libpulse0,libfdt1,libpixman-1-0,zlib1g,python3,libspice-server1]

your snap is being executed on top of the (readonly) core or core18 snap, not on top of the OS itself … this makes sure a snap can be literally run everywhere (where snapd can run), but requires you to include libraries you use via the stage-packages directive in your snapcraft.yaml.