Hi Sanchit --
As you'll see, there are bugs from a wide variety of projects, including but not limited to Firefox. A fact of software is "there are always bugs", so there's plenty to do on Firefox, too. You'll note that not all of the "bugs" in Bugzilla or GitHub are actually flaws -- many are ideas for improvement (enhancements) or just things that need to get done (tasks).
Thanks for your interest in contributing to Mozilla!!
Dustin