Well, I can list a couple of things off the top of my head. For reference, I use both Chrome and Firefox on Linux at work and at home, and I have Chrome exclusively installed on my windows partition at home. I am trying to keep my windows install at home light, so I install as few things as possible. Also, I picked Chrome for windows because I wanted to get used to it more.

1. My biggest usability preference for Firefox is the 3.0-and-above address bar in Firefox. I discovered it by accident, but it has probably had more effect on my browsing of the web than any other recent browser feature. With it, I almost never have to fuss with bookmarks or type in the full URL of a web site. The address bar is absolutely amazing at guessing where I want to go based on one or two simple keywords. Chrome, on the other hand, always seems over-eager to give me the google search results for what I'm typing, doesn't handle things like out-of-order/non-contiguous keyword matching as well, doesn't prioritize previously visited web pages high enough, and doesn't order shorter matching prefixes on top, among other things.

2. As a computer-savvy person, I prefer firefox's more standard interface of having a menu bar on top. I also find it easier to configure Firefox, as again, it's editing windows work like most other programs I'm used to. That's not to say I don't like any other alternative UI's. I love, for instance, the ribbon interface in Office 2007 and I still feel like I can find my way around in it.

On a similar note, I also don't really like how Chrome's configuration is managed in separate tabs in web pages. I prefer the again more standard modal dialogues, separate windows, and native style widgets that Firefox uses. I feel like this appropriately separates Firefox-specific things from the rest of the web.

I see a major difference in purpose here between the two browsers. Firefox wants to fit in with the rest of your OS. Chrome wants to pretend your OS doesn't even exist.

3. I feel like I can manage Firefox bookmarks much better. I prefer, for instance, that added bookmarks not go to the bookmarks bar by default. Also, if I want, I can always drag and drop the current tab onto the primary bookmarks bar if I want to. Lastly, because of the location of the bookmarks menu, I know I can easily access bookmarks with alt+b and then use the arrow keys to find my bookmark, or I can even hop to bookmarks based on their first letter. This essentially allows me to access all bookmarks without even touching the mouse.

4. Firefox's search interface feels nicer. I like how the search bar shows up at the bottom left and spans the entire width of the screen. I believe it is less likely to be intrusive at the bottom as a lot of pages have right-columns. The fact that it spans the entire width makes it easier to notice the bar in the first place if you are new and have no idea where the search bar would show up.

Also, I like how options like case sensitivity and highlight-toggling are immediately visible, how alt+n and alt+p move forward and backwards, and how all of these and other options have underlines as the standard affordance for their alt-shortcuts. I know that enter/shift+enter work for both browsers for navigation, but there is no affordance for that. Lastly, for the VIMer inside me, Firefox responds to / for quick find! If that isn't cool, I don't know what is :).

There are probably other things, but these stick out at the moment.