Firefox is changing with v10. It will be considered a long term release. Those that want to stay on the fast path, will be able to move to v11, v12, etc. The fixes will be back ported to v10, which will become v10.0.1, v10.0.2, etc. I'm not sure when they'll push the v10 users up to a newer release, but I believe you'll get at least 18 months on v10 before being told to change. This change came about because companies have been complaining loudly to Mozilla about this rapid release cycle.
Interesting, gonna be a splintered browser soon, what a mess this type of versioning will cause headaches for many.