More generally, FirefoxOS does not automatically get the latest version of Gecko, because the process for mobile operating systems is more complex. The operating system must first pass certification by (if I recall) both the constructors and the operators. This operation is both long and costly, so it is probably impossible to follow the Firefox desktop/Android 6 week release cycle for the time being.
However, if I had to guess, I would assume that FirefoxOS will receive about one big update every few months, with all the new features and optimizations of Gecko. I know, for instance, that some of the optimizations I have added recently to Gecko have missed the release train for the next version of FirefoxOS (I believe that's still 1.1) but should be part of the release after that.