Been all around the block, and always come back to Firefox... I love it for its add-ons, which don't slow it down overly, if I'm careful about which ones I install. And since around v9 or 10 when they finally fixed its tendency to take 20 minutes to quit properly, it now seems very smooth and quick.
Opera used to be the fastest thing available, before v11 I think when it became a RAM hog like FF pre-v9. Chrome I liked because it syncs so nicely with Google accounts... and sync'ing also syncs add ons, but I didn't find it especially fast.
The main thing was that my favourite download manager (FDM) never seemed happy with Chrome or Opera. Especially for downloading vids from Youtube etc. So, back to Firefox.