These fonts are intended as a system font replacement on Mac OS X 10.10 Yosemite.
They are based on the Fira Sans font family and are licensed under the Open Font License (OFL) 1.1 or later. This package has been prepared by Jens Kutilek https://github.com/jenskutilek/FiraSystemFontReplacement.
Download and unzip the font files from http://www.kutilek.de/download/fira-system-fonts.zip.
Copy the 3 font files into the /Library/Fonts folder on your system disk.
You may have to log out and in again in order to see the change take effect.
Delete or move the font files from the /Library/Fonts folder.
The baseline of the bold font is too low in the menu bar.
These Fira fonts have a special name table with names identical to those of the system fonts. Because the font folder /Library/Fonts takes precedence over the fonts which are in /System/Library/Fonts, these specially crafted fonts are used for the user interface instead of the real system fonts. The original system fonts are not deleted or modified in any way.