I wonder if at this point Google wants to keep Mozilla in the game. Their market share is low enough and it keeps antitrust regulators at bay. It looks bad if there are only two major browsers, one of which owned by a company which already got slapped for this very kind of monopolistic behavior.
With Firefox 57, the UI changed to look almost identical to Chrome. The tabs are now boxy, not round anymore. The hamburger menu changed from the useful big box with big icons to a long menu that looks 99% like Chrome incl the unusual page zoom buttons in the menu. Now we may know why, Google is paying them again, after Yahoo deal is no more.
Sorta like Intel and AMD.