What sets a browser apart from other is I think its extensions and performance. Firefox and Google Chrome are the best examples. Both of them have a highly active community for building extensions and you have over thousands extensions to choose from.
Yes, extensions give additional features and functionalities to a browser, but the question is how to build an extension.
If you’re really interested, here I am presenting an e-book (currently available for free) titled ‘Build Your Own Firefox Extension’ that makes you to build a custom extension for Firefox.