Firefox because of the debugger and add-ons. Chrome, on the other hand, is faster and doesn't require so much RAM. I use both Firefox & Chrome. But my all time favorite is Chrome.
The best browser is for SEO is Google Chrome. Because Chrome has a facility of Chrome extension, it means we can set our popular tools extension on Chrome easily and use it any time without login. And Google also has facility of show the rank in plus and minus nature by SERP Trends.