[CSS] flex-basis not respected in Firefox

There is no JavaScript used to style this site, to preemptively rule out that question. There is a only single statement used to remove WordPress-inserted breaks in an inline article element.

Here is the site in question, my personal blog: http://www.bhalash.com/

I recently launched a rebuild of my personal blog, and just because, I've gone with flexbox all the way up. I've been bashing my head off a single problem all day: the flex-basis of the #content element is not respected by Firefox and a minimum width appears to be applied. I'm aware of a log of bugs and quirks related to flexbox cross-browser, but nothing I've found matches this particular issue.

Expected behaviour: Content column should neatly scroll in lockstep with the fixed sidebar until it reaches the breakpoint at 1024px width and switches to a column layout. #content's width should be either 75% or 66.6% of the parent div, #site.
Actual behaviour: Works normally above ~1300px window width. Below that #content's width remains about 80% that of the window until the 1024px breakpoint is reached. This occurs on load too; the page seems to render correctly until the DOM finishes loading, at which point it jumps from expected width to 80% width.

Expected (Internet Explorer 11): https://i.imgur.com/hjym5FV.png
Firefox actual (Firefox Developer Edition 40.0a2): https://i.imgur.com/qVvL75w.png

Compiled and with webkit prefixes and breakpoints (the behaviour occurs independently of any breakpoints) removed, the CSS looks like this:

#site.side-by-side {
    display: flex; 
    flex-direction: row-reverse;
}

#header {
    position: fixed;
    top: 0;
    right: 66.6%;
    bottom: 0;
    left: 0;
}

#content {
    flex: 0 1 66.6%;
}

#header is a sibling of #content, but #header is fixed instead of having a flexbox layout. I still had this problem after I gave #header a flex layout-#content won't go below a minimum size and #header instead moves behind it.

TL;DR: #content seems to have an invisible min-width applied by Firefox and I can't figure out why.