I think clearly Google want Firefox alive as a 'competitor', and they explicitly are buying that Google search is the default. I highly doubt they have any agreements limiting Firefox's market share or features though - that would undo the benefit of it being a competitor if it ever came out, but more significantly they don't have to. Mozilla have managed to achieve all that on their own. I actually think Google would probably rather that Firefox was at say 10% market share so they had a more legit argument that it was a competitor.