Actually, it makes much more sense if Google pays Mozilla to maintain an alternative that never becomes truly competitive.
I don't think actual competition benefits Google in a commercial sense. If we considered the situation purely rationally, Google's most logical decision would be to use their budget as leverage, threatening off the books to prevent any strategy that might make Firefox "viable again", under the assumption that Google focused primarily on market share, while Mozilla focused purely on survival.
Yes, Google definitely has an incentive to keep Firefox inferior. If it became a real competition, that money flow would likely stop.
However, if Firefox drops to ~0% usage, it will definitely stop, as that ruins Google's monopoly defense, which is the motivation for it! Firefox usage is supposedly already as low as 2.33%.
I don't think actual competition benefits Google in a commercial sense. If we considered the situation purely rationally, Google's most logical decision would be to use their budget as leverage, threatening off the books to prevent any strategy that might make Firefox "viable again", under the assumption that Google focused primarily on market share, while Mozilla focused purely on survival.