YMMV. Most builds will probably be ok but there's always the chance that something will be broken.
I don't know if Firefox has something like Chrome's release channels but on Chrome the beta channel is pretty damn stable and doesn't get updated on a daily basis. If Firefox has something like that I would probably recommend that over nightly builds. If you want to be on the bleeding edge then I would recommend the most bleeding edge channel that isn't doing daily builds if such a thing exists with Firefox.