There's a really sharp drop after August 2020. It looks like the former devs increased their activity a bit in February-March of 2021 but after that is almost flat.
A browser engine is something way too big to sustain by hobbyists or as a side job. Most of small user base browsers are Chromium derivatives, WebKit wrappers (iOS).
There's forks of Firefox like Waterfox or Palemoon which end up slowly getting behind in features or security implementations.
https://twitter.com/directhex/status/1293352458308198401