That's a valid question. Unfortunately, it's difficult to quantify.

The state of browsers in general has been a moving target since NCSA Mosiac; about around 1993 or so. So the last three decades has been a ceaseless grind of new features, security enhancements, performance enhancements, and so on. And this feature set is absolutely monstrous in scale, as it includes backwards compatibility to most of those features (if not all of) back to that beginning over 30 years ago. So, work on any browser is by definition perennial, and it only ever gets more complex.

For Firefox, well, just take a look at their bug tracker. It's broken down by component, but each link on this page is its own fresh hell of things to do, many of which are barely a year old: https://bugzilla.mozilla.org/describecomponents.cgi?product=Firefox

I would also argue that the only other software projects that compare to a web browser in terms of sheer scale, compatibility, and longevity, are things like the Linux Kernel or maybe the entire Microsoft Office suite. IMO, software in this class is a lot of work to keep going, no matter how you slice it.