Hm, I'm unable to reproduce this error with fontmake 1.6.1 and the latest version of FiraCode from https://github.com/tonsky/FiraCode. My output:
$ fontmake -g FiraCode.glyphs INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source INFO:glyphsLib.classes:Parsing "FiraCode.glyphs" file into <GSFont> INFO:fontmake.font_project:Building OTF for FiraCode-Bold INFO:ufo2ft:Pre-processing glyphs INFO:ufo2ft.filters:Running DecomposeComponentsFilter on FiraCode-Bold INFO:ufo2ft.filters:Running RemoveOverlapsFilter on FiraCode-Bold INFO:ufo2ft:Building OpenType tables INFO:ufo2ft.outlineCompiler:The copyright was normalized for storage in the CFF table and consequently some characters were dropped: 'Copyright Copyright 2015 by Nikita Prokopov' INFO:fontmake.font_project:Saving master_otf/FiraCode-Bold.otf INFO:fontmake.font_project:Building OTF for FiraCode-Regular INFO:ufo2ft:Pre-processing glyphs INFO:ufo2ft.filters:Running DecomposeComponentsFilter on FiraCode-Regular INFO:ufo2ft.filters:Running RemoveOverlapsFilter on FiraCode-Regular INFO:ufo2ft:Building OpenType tables INFO:ufo2ft.outlineCompiler:The copyright was normalized for storage in the CFF table and consequently some characters were dropped: 'Copyright Copyright 2015 by Nikita Prokopov' INFO:fontmake.font_project:Saving master_otf/FiraCode-Regular.otf INFO:fontmake.font_project:Building TTF for FiraCode-Bold INFO:ufo2ft:Pre-processing glyphs INFO:ufo2ft.filters:Running DecomposeComponentsFilter on FiraCode-Bold INFO:ufo2ft.filters:Running RemoveOverlapsFilter on FiraCode-Bold INFO:ufo2ft.filters:Running CubicToQuadraticFilter on FiraCode-Bold INFO:ufo2ft.filters.cubicToQuadratic:New spline lengths: 1: 266, 2: 2368, 3: 740, 4: 188, 5: 23 INFO:ufo2ft:Building OpenType tables INFO:fontmake.font_project:Saving master_ttf/FiraCode-Bold.ttf INFO:fontmake.font_project:Building TTF for FiraCode-Regular INFO:ufo2ft:Pre-processing glyphs INFO:ufo2ft.filters:Running DecomposeComponentsFilter on FiraCode-Regular INFO:ufo2ft.filters:Running RemoveOverlapsFilter on FiraCode-Regular INFO:ufo2ft.filters:Running CubicToQuadraticFilter on FiraCode-Regular INFO:ufo2ft.filters.cubicToQuadratic:New spline lengths: 1: 257, 2: 2313, 3: 783, 4: 208, 5: 25 INFO:ufo2ft:Building OpenType tables INFO:fontmake.font_project:Saving master_ttf/FiraCode-Regular.ttf Am I running fontmake with different arguments?
