I quite like swiftfox, the developer does not pretend that it's not an optimised firefox, in fact he attributes a great amount of credit to firefox (and rightfully so).
The optimisations do, in my opinion, create superior browsing speed and with compatibility for plugins and all else, it still has the advantages of firefox.
I would say swiftfox is to firefox what linux mint is to ubuntu.
The issue of licensing is debatable, the article pointed to does not credit Jason Halme for making the source available on the products webpage.
Anyhow, one feature of swiftfox which I do not like is that I had both swiftfox and firefox installed, some time later if I try to open firefox, it says it's starting - and then swiftfox opens.