Here is Dylan talking about the Viewer List. They use a font that only Chrome can display so when you look at the Viewer List in Firefox it looks different than it does in Chrome. And Dylan admits he hasn't looked at the Viewer List in Firefox. I'm assuming Safari and Internet Explorer also get short shrift, not to mention my alternate browser, Opera.
Here's an idea that most Web developers already know. Design your site so that it looks good and works in current and recent versions of all of the major browsers: Chrome, Firefox, IE, and Safari.
I highlighted the relevant section from Community Chat, which you can see here.
Update August 19: Dylan added a comment to the video highlight saying what fonts the developer chose which "unfortunately" aren't supported by Firefox. I feel like I'm talking to a wall with these guys at times. YES, that's the point. Fortune plays no role here. The developer elected to use a display font that Firefox can't render. The developer either doesn't know what he is doing or he doesn't care that 30% of the site viewers (or whatever Firefox's market share is) won't see the site as intended. It's not that difficult: choose fonts that all the major browsers can handle.