I made this fork to quickly test something out. Basically I had a testing env in another project which had firefox v89 installed and I was getting window.TestingLibraryDom is undefined for the testing library I wanted to use. I wanted to see if this error only exists for the particular version of firefox I was using so I ran this library on firefox 94.0.1 in this repo... and unfortunately I continue to see the same errors. Seems related to testing-library/nightwatch-testing-library#31 though that github issue was closed (might need reopening to fix this.)
Output of what you can expect can be found in ./Error_Output.PNG file in the root folder.
#EDIT (11/14/2021):
A solution to fix the firefox issues was tested against this fork now seems to work. Mentioned more in deph at testing-library/nightwatch-testing-library#31.