I used Firefox a lot, since the times of Mozilla M3. Over the years it added a lot of innovation, but it became a heavy user of resources. When a lot of tabs are open it shows, and stability is sometimes a problem.
When Chrome came out it was doing only a few things, but those were very fast - and tabs are different processes for added stability. I was in the first beta and still use it, sometimes switching to Firefox when some extension I might need is non-existant on Chrome.
As Gmail is quite faster on Chrome I decided to switch. Added bonus is favorites sync, but also Firefox has that.
I suppose that Firefox - and even Internet Explorer - are catching up, so using either is not going to make a lot of difference in the long run.