Can confirm it works as expected. @kyrios123 Let's get this added to the Desktop Actions in files/firefox.desktop for your 64.0 patch.
Dec 12 2018
Dec 11 2018
You're right, by default, --new-tab has a strange behaviour.
However, if you run firefox --new-tab about:newtab, it runs as excepted.
Dec 10 2018
Unfortunately it does not appear that --new-tab in fact opens up a new tab, at least in 63.0.3-145, whether it's using --new-window to open up the instance or not.
Assigning JoshStrobl to investigate and triage. Referencing T7289.