Why are they spending money on infrastructure and support but getting no revenue in return?

I already addressed this in my comment. If you want me to expand on how they most definitely can make money from something like this, Mozilla:

• Gets revenue from their paid VPN service that already exists, and it would be a way to convert users to a revenue source, since the thing being taken away after the data cap is itself a VPN
• Gets donations, which more users with a good opinion of the browser will bring
• Has sponsored integrations, which pay money on a per-click basis, (e.g. AccuWeather integration where Mozilla gets paid if you click through to their website, pinned sites like Amazon that appear on the new tab page for new users) and ones that are influenced by overall number of Firefox users (e.g. Google’s deal to be the default search engine when you first install Firefox)

If this feature brings in new users, they can get revenue from any of these 3 sources, especially the sponsored listings. If this feature is just a benefit for existing users that might have already changed all their defaults and disabled sponsored content, it increases the chance of VPN conversions and donations, and increases the likelihood someone will recommend Firefox to a friend.

Either they are okay with losing even more money, OR they plan to enshittify.

Or they’re trying to get and retain users, which helps them make money from existing revenue options without having to make anything worse, while also providing a beneficial feature. I’m not saying there’s no chance they’ll enshittify, but I don’t think unconditional pessimism is the right move here.

For this and many many other reasons, it’s time to switch to a privacy fork like LibreWolf or WaterFox

I can’t speak to Waterfox myself, but I would agree with saying LibreWolf is a good idea if you care.

I just personally haven’t bothered switching since Firefox currently works fine for me, and anything they’ve done I dislike is fairly easy to just disable in settings and never see again.