Their problems aren't problems so to speak, it's their perspective. Firefox is fine if you're used to it, but Chrome blows it away in speed by such an astronomical amount that it makes it seem like something is wrong with Firefox, which is what most people who have "issues" with Firefox feel. The first startup of Firefox takes something around five times as long as Chrome. If one tab in Firefox is running a difficult to handle website or even loading a large website, the entire browser and all of your tabs will be laggy; this doesn't happen in Chrome. Chrome completely destroys Firefox in Flash and Java performance. A test anyone can try is to run the web game Lord of Ultima in Firefox and then do it in Chrome. The former will be under 30fps and choppy and the latter will be 60fps smooth as glass.
They aren't having problems, that's just how Firefox is. To some people Firefox's behavior is acceptable(mainly to people who are used to browsers working that way), to others it is not.
*This is not to say Firefox doesn't have some advantages on Chrome, it does, but speed is not one of them.