It will hurt but it won’t be their death.
They have been saving up a bit last year if you see the financial reports their reserves are just above $1B now and there are others who paid in the past (like Yahoo did till 2017) who will pay Firefox a decent amount if not like Google does .
My guess it is likely be Bing or probably a new generation AI company like OpenAI who will replace Google and perhaps even pay similar or close to what Google pays. The traffic is worth a lot. Bing attested to click flow as the reason they cannot make a better product in their testimony in this trial.
Also Google will either be allowed to continue the contract till its current end (I believe 1-2 more years ) or will pay fully and release Mozilla from their obligations (Mozilla is not party to the case so early termination without compensation would be penalty on them for no reason ).
Mozilla will need to make some significant cuts and layoffs no doubt will be hard on the team, but the product will survive.