Firefox has hit a new landmark today with the release of version 40. The biggest change is improved Windows 10 compatibility, though previous versions of Firefox certainly work fine with Microsoft's new OS. Instead, the changes are mostly aesthetic to bring the browser in line with Windows 10's design and make it easier to use on a touchscreen. For instance, the "close" buttons on tabs are now bigger so that they're easier to use on a touchscreen, and the address bar has bigger fonts. It also has revised tabstrips and toolbars plus a custom Windows 10 theme.
Source: Mozilla