With the new release of Firefox (v 43), there are now 2 versions. One for 32 bit computers (which is what it's been to date) and another for 64 bit computers. Mozilla say that for those with 64 bit computers, Firefox should give better performance. It's not clear if the upgrade to 64 bit computers is automatically done or if you have to download it.
An article on PC World gives a bit more info on this. http://www.pcworld.com/article...-era-on-windows.html
The article does warn that plugins such as Java and Silverlight do not and are not intended to work on the 64 bit version. It says that the Flash plugin will continue to work due to its popularity.
To download the 64 bit version go to https://www.mozilla.org/en-US/firefox/all/
There are both ordinary and 64 bit versions for Windows and also Linux. So one can go back to the ordinary version if the 64 bit one causes problems.