The Mozilla numbers show mobile devices will outnumber desktop devices. The Wikipedia numbers show web usage. People surf more on desktop machines, and don't (need to) upgrade them as much so both figures are correct.

The real issue for Mozilla is that you can get great benefits (shared history, passwords etc.) if you use the same browser on both platforms and sync them. Luckily Apple and Microsoft's lock-in approach seems to be holding them back here, but I'd imagine Android Chrome will be a big driver for Desktop Chrome (and possibly vice versa).