Normally say a version like ex: 34.0.x would be a update for security and or stability fixes. In this case the 34.0.5 is not a update and is essentially the same as 34.0. The only difference really is that the en-US 34.0.5 has Yahoo as the default search engine as Google is no longer the default though still available as a search engine. It was a way they decided to differentiate between the two 34.0 Releases. If there is chemspill updates before 35.0 Release the two may likely be 34.0.1 and 34.0.6 https://www.mozilla.org/firefox/34.0/releasenotes/ https://www.mozilla.org/firefox/34.0.5/releasenotes/ https://blog.mozilla.org/blog/2014/11/19/promoting-choice-and-innovation-on-the-web/