It is "TheNextWeb" here, so not exactly deep journalism. Its true that if Google declined to support Mozilla that Microsoft could get in there for less than the $300M that Google paid, and it would probably push Bing closer to parity with Google in terms of searches done.

So more accurate reasoning is "Search advertising is the best, Mozilla drives a lot of traffic, any search engine would love to have 'control' over Mozilla's choice of default search provider and how that works to insure they get the traffic from Mozilla customers."

So no, Mozilla doesn't need a 'backup' plan unless Bing goes away.