First, HTML5 videos. I watch a lot of lectures on Youtube, and HTML5 videos have a speed option that most lectures benefit from (30min instead of 1 hour lecture). Sure, Firefox plays HTML5 too - but not as many. Some options are not available.
Second, Flash on Firefox has been *horrible* at least for me lately (I have the latest version of everything, Windows 7 system). After the latest Flash update all I have to do to crash the Flash plugin is right-click over a Flash area. And it's been crashing a lot for me for a long time.
On the other hand, (from a user point of view, not web developer) I often run across bugs in Chrome while the same doesn't happen (to me) with Firefox. So if I could I'd stay with Firefox.
I think as a web developer, especially when you develop modern apps and not just intranet enterprise apps (that are very conservative in what functions they use) Chrome may be more tempting at this point. I'm guessing - I only develop those "boring" apps where the intelligence is in the business logic and on the server and I don't need to do as much in the browser.