Weirdly sloped font in some apps after 16.04 upgrade

[Adam Funk]

On 2016-08-26, Ralf Mardorf wrote:

> On Fri, 26 Aug 2016 13:09:39 +0100, Adam Funk wrote:
>>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?

> what font should it be, regarding the HTML source? In Firefox at the
> opened website push Ctrl+U to see the page source and then Ctrl+F and
> search for      font      , since I don't have Thunderbird installed, I
> don't know how to view an email's source by Thunderbird, but usually
> mailers provide such an option, too.

Looking at the source and the "inspect element" stuff, it seems to be this:

   font-family: Arial, Helvetica, sans-serif;