The economy gets worse. Ad spending goes down. Google's search revenue decreases. Mozilla's search deal with Google is less profitable.
Further I do not believe their Search deal is dependent on Advertising at all, it is about number of searchs, and being the default not a % of ad spend
Iain and I are both Mozilla employees, and terms of search deals are very much NDA-covered things -- we can say very little about our limited knowledge.
All I will say is that the revenue from Google is not fixed, and what the press release said was in fact true: revenue was substantially affected by COVID.
Revenue is down two consecutive quarters. It is well-known that the vast majority of Google's revenue comes from ads (see eg https://www.forbes.com/sites/greatspeculations/2019/12/24/is... or https://www.theverge.com/2018/10/25/18024654/alphabet-google...)