I have been a loyal Firefox user. I moved away from IE to Firefox way back when I got my first mac, many years ago. The main reason at the time was to have a consistent browsing experience between PC and mac. I've been a loyal user for many years, enjoying each new release.

When Google came out with Chrome, I briefly looked at it and even though the speed was great, lack of extensions at the time kept me in the Firefox fold. I was itching to move over but ubiquity was the main reason I was still sitting still. It was a matter of time however, especially since ubiquity has now been demised and behaviour is more and more erratic with each new release.

I have now moved over for good. The features that made me change over are:

• Integrated PDF viewer. That has been the real trigger. The plugin is still a bit rough on the edges and I still need to disable it once in a while (by going into chrome://plugins) in cases where saving a PDF is just impossible for some reason. However, when it works, it's great. The rendering is fast, and as close to web-like as you can possibly have with a PDF file. While you can have a similar experience by using a Google Docs extension in Firefox, it's not as fast nor does it work on private intranets. It's only available in the beta channel for now but it's just a matter of time until it arrives into the mainstream product. 
• Real drag and drop. One of the great feature of Gmail is the ability to drop files in the UI to attach documents. With Chrome, that behaviour works even for regular sites. In most sites where you have an upload button, you can just drag and drop a file onto the button instead of opening the file dialog. A real time saver. That's probably the next best feature for me.  
• Automated extension synchronization. That one is also a great time saver. If you have many computers, having to install your plugins in each one is really a pain. With Chrome you just have to do it once and all computers are automatically synced. Just login and all your favorite extensions appear!
• Speed. While this one has been touted as the major benefit, and while it's been described as the main reason why you should move over, it was not enough for me. Still I do appreciate the speed and the process isolation that Chrome brings to the browser scene. 

They are many other features that will warrant the move for you, but for me those four were the ones that really made a difference.