Once again Mozilla is fiddling with Firefox’s UI with Firefox 40 due for release on August 11th. However, it will only affect Windows 10 with this release (Firefox will auto-detect if it is being run on Windows 10 and apply this theme). Windows 7, 8 and 8.1 will be getting the theme updates in a later version.
With Firefox 40 on Windows 10…we’ve matched the tabstrip and toolbar to the native Windows 10 theme. This includes refinements to our standard icon set, as well as much improved HiDPI (>1dppx) support. All of our first-tier icons now have 2× variants that are shipped with the browser, and the remaining icons buried in the depths of the browser should be fixed soon as well.
Also, the font on the URL/Search Bar will be increased slightly, but the size of the actual toolbar has not been changed.