In version 3.108 of Fira, the sTypo metrics (sTypoAscender, sTypoDescender, sTypoLineGap) have been set to 785, 215, and 200, respectively, for a total line height of 1200 (i.e., 1.2 ems). This is good-- this does indeed match what was present in version 2 of the fonts.

However, at least two major OSes' font renderers do not use the sTypo metrics at all-- in particular, Windows GDI uses the usWin metrics consistently, while Mac OS X uses the hhea metrics consistently, regardless of the value of the USE_TYPO_METRICS flag. (See http://typophile.com/node/13081 for further discussion of this.) DirectWrite on Windows does obey the sTypo metrics--but unfortunately, there are still some Windows machines (such as my work laptop) where DirectWrite's rendering is very buggy, forcing the use of GDI.

The total line height as measured by both of these sets of metrics should be equal to the total line height from the sTypo metrics. Unfortunately, in the case of Fira, they're not. With an ascender of 1050 and a descender of 500, the total line height as measured by these metrics is 1550 (1.55 ems). As a result, line spacing is not identical across OSes.