[FIX] website: responsive images in table for Firefox only

It looks there is a bug in Firefox concerning responsive images in table. See bugzilla https://bugzilla.mozilla.org/show_bug.cgi?id=975632
Bootstrap advises to use width: 100% for .img-responsive as workaround were needed.
The @moz-document is to apply this for Mozilla only.

opw-617582
opw-618659