The end of Firefox
If search engine presets in browsers will be illegal, that will be the end of Mozilla and Firefox, as >99% of all revenues fo the former is from Google - or whatever search engine pays it for being the default. Now,
Mozilla could still earn some money through what's in essence search referral royalties (unless that's also plain out outlawed), but once it can not charge search engines directly for being the default, and the choice will lay with the user, the searches themselves will be worth a lot less, and Mozilla will only earn a fraction of what it does now.
Also, of course "the end" only means "as we know it", and that Mozilla will most likely change its business model, and will most likely become itself a kind of "ads dealer" or "ads marketplace", and will sell the ads it shows in the browser directly.