There must’ve been a flexbox bug in Firefox 45. Today, it was brought to my attention that nav was getting cut off on a site. I added a flex-shrink: 0 to ensure the logo shrank to accommodate, fixing the problem in Safari, but not Firefox. Thinking it odd that Firefox was behaving so differently from other browsers, I decided to check for an update, and 46 happened to be available. After updating, the problem disappeared. I’ve ran into browser bugs with flexbox before.