That, sadly, this will only happen to US users... by default, and by default means that anyone can still switch to Google, and also that Google is over a 90% of usage in EU, therefore the deal is not too much for Mozilla, but is true that they will have less money. In US they have about 75% of share.
Anyway, nowadays, whatever that removes portions of power gave to Google, is great!
Stats from Statcounter for EU and US:
http://gs.statcounter.com/#search_engine-eu-monthly-201310-201410
http://gs.statcounter.com/#search_engine-US-monthly-201310-201410
And about Mozilla, I don't think they will suffer much. If they changed for Yahoo is because they put a good amount of money on the table to sign for.