Weirdly sloped font in some apps after 16.04 upgrade

[Adam Funk]

I just upgraded a desktop machine from 15.10 to 16.04 (a little late,
I know!).  Everything is OK except for one weird problem: a lot of
web pages in Firefox and some e-mails in Thunderbird are displayed in
a strangely sloped font --- like backward-italic, i.e., up to the
left.  I have no idea where to find whatever is causing this, & it's
pretty hard to google.

Any suggestions?

Thanks,
Adam