Based on the 64-bit code included in the main Firefox tree which has not been considered stable enough for release builds by the Firefox builders. Firefox has

Based on the 64-bit code included in the main Firefox tree which has not been considered stable enough for release builds by the Firefox builders. Firefox has had 64-bit compilation capability for a long time. I think Firefox 42.0 - these builds are uploaded now to mirror for tomorrow's release - will be the first time they do an official release version. Note that until they are announced, you shouldn't start using these builds.