On Monday, December 1st, Mozilla did another “strange release”. They did this last month with Firefox 33.1 aka The Anniversary Release. So for December they released Firefox 34.0.0 AND 34.0.5. I think I understand the differences, although most people on the forums are still scratching their heads trying to figure this one out.
For the United States (en-US) and Russia (RU) builds, user have the option to changing over to the default search engine upon first run of Firefox 34. For en-US users this is Yahoo and for RU users this is Yandex. Since this change is optional, user who update from an earlier version of Firefox (33.1.x or 33.0.x or older) will be updated to Firefox 34.0.0. Users who download and install a fresh copy of Firefox will have Firefox 34.0.5 which for the en-US and RU builds will have the new Yahoo and Yandex search engines as default.
The next planned release is Firefox 35 on or around January 6th, 2015. However, between now and then if there is a chemspill (something major broke and we can’t wait until the next scheduled release to fix this) release it should be Firefox 34.0.6 (though it could be 126.96.36.199).