[EDIT] I'd actually be a lot happier with this if FF were more aggressively adding new Internet or alternative-Web protocols, or trying to use their platform to give things like federated social networks more visibility and better prominence (via, say, optional but tastefully promoted integration with the browser). That might also, you know, differentiate them and give me a reason to recommend it to non-nerds again, despite "it's way, way faster than anything else, and lighter on resources, and blocks pop-ups and has tabs and doesn't have an ad banner!" no longer being enough (or even true, for some of those qualities) like it was in the early days.
I’m sure someone will correct me, but without looking at the source code, I suspect the following will have played a role in increasing code size since the earliest days of Phoenix:
* support for newer media formats, from MP4 through AVIF
* DRM maybe?
* WebGL
* SVG?
* The complexity of modern CSS
[EDIT] in fact, some things browsers do now were seen as terrible ideas when Phoenix was released, because they would kill performance, since they're so much more expensive than alternatives. Vector graphics were regarded as to-be-avoided for a long time, because they're so much more taxing on the client than raster. CSS doing animations would have been considered insane, fit only for "look how I can bring your machine to a crawl with a web page" demos. Maintaining an entire "shadow DOM" of Javascript objects would have been regarded as parody-levels of resource wasting for all but very niche use-cases. No wonder the modern web is so slow and resource-hungry....
Mine would be somewhere between 0.5FTE assuming the bare minimum of keep-the-lights-on QA testing for each release and ongoing bug triage up to maybe 3FTE if it requires periodic security and design updates to keep up with changing standards/platforms/protocols.