If I recall correctly (from experiments, years ago): after successful installation of the Widevine plug-in on Linux, it should be possible to copy the relevant file(s).
Place the copies in your preferred Linuxulator.
Placement alone may be insufficient. Also, it may be necessary to set advanced preferences in Firefox (for the application to believe that installation was automated, not performed by hand; that type of thing).
(This is not to understand what's wrong, neither does it aim to fix whatever is wrong, but it's a possible workaround.)
I haven't the time to gather more details at the moment, sorry. Maybe later …
I'm quite certain that doing so will not allow installation of Widevine for Firefox.
The differences between Chromium-based browsers and Mozilla Firefox are immense.
A nice idea, however in a case such as this it is (with respect) not useful to follow that train of thought.