This is something we noticed in Bug 1424095: <iframe> elements have `display: inline` in xul.css and then also have `width: 100px; height: 100px;` in each global.css file (i.e. https://searchfox.org/mozilla-central/rev/062e1cf551f5bf3f0af33671b818f75a55ac497b/toolkit/themes/linux/global/global.css#81-89). We need to override width and height in XUL flexbox emulation (Bug 1424095). If it's possible to stop setting them in the first place that would be simpler. But this needs some investigation since I'm not sure if there's a reason they are set - the CSS landed and has remained unchanged since bug 247631 / https://github.com/mozilla/gecko-dev/commit/e5632337c122eac31fbc85d2a5cba910448733fe.