How big a security risk is it, really? Firefox Mobile apparently has a 0.66% usage share, which is lower than desktop Linux, and desktop Linux anecdotally is pretty much a non-target for malware authors. I, for my part, blocked my FF Mobile autoupdates upon reading this and am not going to lose a lot of sleep over the possibility that some malicious website would choose to target me over the Chrome/Safari users.

(I would also suspect, without concrete evidence, that an average default mobile browser user has more valuable data on their phone - Firefox seems like the browser of choice for the same sort of refuseniks who would still do most of their online banking and what-not on PC only rather than installing their bank's app)