Bump the minimum Firefox version to 25 in anticipation of the sub-worker from frameworker requirement

Romain is currently working on using sub-workers from the frameworker for the SPA work. In anticipation with that and due to current activities, I think its a good idea to bump our requirement to FF 25 now, rather than in a few days time (we probably also need to improve our browser detection & UI clarity, but I'll file separate cards for that).