pptr-ff: Black screen when building FF

[SlootSantos]

@aslushnikov Yesterday I reported in #3802 that the firefox build failed due to changed behavior of the xcode-select --install command.
I could actually workaround the issue and added the tipp to the readme.

Turns out FF has a known issue running the browser built with MacOSX SDK 10.14.
Described here: https://bugzilla.mozilla.org/show_bug.cgi?id=1493330.

A couple of days ago Mike Shal from the mozilla team added a check to the build process looking for the used SDK.

The guys over at mozilla helped me figuring out how to build it on Mojave.
In my opinion this is actually quite tricky to figure out on your own.
Hence I will add yet another PR to adapt the documentation!

Sorry for the confusion about how to actually build FF on Mojave!

[SlootSantos]

Added PR: #3813

[aslushnikov]

@SlootSantos thank you for figuring this out!