I find it kind of weird that you’ve had the tabs on the bottom. Every time I install Firefox, they’ve always been on top, from version 1.0 and up, they’ve never been on the bottom for me, never even seen the option for them to be on the base.
As for the switch from FireFox to Chrome, it’s a consideration of mine as well. Firefox since 3.0 or 3.5, has been heavily bloated at the best of times and very unstable the worst of times (excluding Flash). They’ve been doing some strange things that has resulted in page load times increasing significantly, app startup can be a few minutes when a lot of tabs are saved, websites that use javascript bloat memory to 600mb+ and the patches break plugin compatibility frequently. I don’t know if a lot of the blame can be placed on the javascript engine, since that was the major change from 2.0 to 3.0, and it’s the engine that keeps getting refined with patches etc. With version 4, they’re dropping the current engine and swapping to a hybrid based on the current engine and the one used by Safari.
I guess the poor old fox isn’t as quick and fiery as it used to be... probably jumped over too many lazy dogs.