[tex4ht] how to make \usepackage{newtxtext} work with tex4ht? it removes "fi" and "ff" from text

[Nasser M. Abbasi]

When I compile this MWE

-----------------------------
\documentclass[12pt]{article}
\usepackage{newtxtext}
\begin{document}
       find differential
\end{document}
-------------------------
The HTML shows

       nd dierential

So the "fi" and "ff" are gone. (looking at the HTML source code itself,
I see strange looking things where these letters are supposed to be,
like small squares with 4 dots in them)

I see these errors/messages

----------------------------
tex4ht.c (2012-07-25-19:36 kpathsea)
tex4ht -f/foo.tex
   -i~/tex4ht.dir/texmf/tex4ht/ht-fonts/
(/usr/local/texlive/2015/texmf-dist/tex4ht/base/unix/tex4ht.env)
(/usr/local/texlive/2015/texmf-dist/tex4ht/ht-fonts/iso8859/1/charset/unicode.4hf)
(/usr/local/texlive/2015/texmf-dist/fonts/tfm/public/newtx/ntx-Regular-tlf-t1.tfm)
--- warning --- Couldn't find font `ntx-Regular-tlf-t1.htf' (char codes: 0--255)
---------------------------------

Complied using
htlatex foo.tex

Also tried make4ht and same result. Tried making math as SVG, no
difference. the "fi" and "ff" are lost.

specs: TL 2015
>which htlatex
/usr/local/texlive/2015/bin/i386-linux/htlatex

thanks,

--Nasser