This issue is not specific to Firefox -- the table generally overflows
on narrow browser viewports (< 1100px). There are some manual pages
where this is even worse, for instance, "Validate filters"[1].

However, this is not a "Documentation problem", but rather a
"PHP.net Website problem". The following CSS might already solve
the issue:

  .doctable {
      display: block;
      overflow-x: scroll;
  }

[1] <http://php.net/manual/en/filter.filters.validate.php>

A closely related issue might happen with embedded images,
such as can be seen on the goto manual page[1].

[1] <http://php.net/manual/en/control-structures.goto.php>

Actually, this is a duplicate of bug #66726.