It's hard for me to remember that far back with any real clarity. At that time it was the best in my opinion. It was set as my primary browser and was my goto browser for diagnostics. However, that browser had some serious problems following standards. So I would probably put its best version a little later than 3, maybe at 7. But then again, at the time the problems of 3 weren't so apparent, other browsers weren't much better.
So maybe. I don't think I was using Chrome much, if at all back then. Was it even around? If it was, it was probably not a lot like what it is now as far as developer tools go. If the choice were between a standards compliant FX 3 and today's Chrome, that would be a tough call.