Same problem here.
On export, Firefox creates passwords.csv.
Attempting to import it to Firefox on a new Win10 system with the
menu item “import from .csv” enabled does nothing.
Importing the passwords to Chrome succeeds; then “Import from
Another Browser” (Chrome, in this case) succeeds in getting the
passwords into FF.
Basically, this “workaround” requires installation of Chrome just
to get your FF passwords from an old system to a new system’s
FF.
From this thread, it looks like this has not been addressed for
>2 yrs.