As far as the Firefox OS is concerned, it could be a case of being the right newcomer at the right time.
Android is free but carriers resent the loss of control over the revenue that Google ultimately makes from the services that run on top of it. This was tolerable when it was Apple; putting up with the same deal from Google and possibly Microsoft is harder to stomach.
Some of this is desperation, of course, given that Telefnica O2 in particular has been under financial pressure in recent times.
"We don't like the fact that one part of the value chain of our business is tightly controlled," Telefnica director of product development, Carlos Domingo told Bloomberg. "In the case of the emerging countries it's worse, because it becomes a monopoly by Google." Ouch.
The other dimension is the user base, possibly the same tech adopters that abandoned Microsoft's IE for the snappier Firefox and changed the future of the browser in the process. Mozilla has never done a smartphone OS but it will undoubtedly be able to build a committed open source fan base from day one.