Opera is my alternative browser of choice. I love Firefox, I really do, but if it weren't for the extensions, I would drop it in a second.
Simply put, Opera is just so much better than it in many ways. Opera has an integrated Adblocker, it looks extremely stylish, there is a lot of screen space so nothing looks clutter, and it has a huge amount of functions.
Furthermore, and what I view as the two best functions of Opera, is the absolutely incredible performance and the amount of features. Simply put, Opera has integrated adblock, session manager, voice talk, and so much more, and all of them are completely useful.
Concerning the performance, not only does Opera open mind-blowingly fast, but I don't think there is any other browser on Earth that can handle tabs like Opera can. Once, I literally had some 50+ tabs opened in Opera, and there was *not a single slowdown whatsoever*. I even opened the task manager and discovered that only 150MB-200MB of memory was being used! Firefox uses that much just by being opened!
As a browser, not only is it feature-rich, but it is also incredibly efficient. If you're looking for an alternative to Firefox, I can not recommend any browser more than Opera!