Comment 5 for bug 1700109

So dont set box-sizing for everything. Why on earth would you think that
is a good idea? Just because the * selector exists in CSS does not mean
it should be used. It will screw up anything that tries to insert any
content or styling in the same context, which viewers do a lot of, since
lots of viewer chrome is actually rendered in the same context as the
content.

If you insist on using it then wrap your content in a
<div class="mycontent"> and use .mycontent *

I can certainly fix the immediate issue by overriding box-sizing on body
but, as I said there will be lots of other issues with injected chrome.