Are <f> and <l> thinner than should be in mono face?

[atheist1917]

Or it's just my wrong perception?

10x

[carrois]

This is a hinting issue. Since Fira Mono and Fira Sans lack manual TT hinting (FiraGO has it), we cannot address the rendering of the single glyphs in detail. All stems have the same width but they render differently in dependence of the individual pixel positions.

[carrois]

No manual hinting in Fira-Mono, yet. Sorry. Thoung, we don’t hint vertical stems anyway. Once you change the the position of text in relation to the pixle grid of your display, you will get a slightly different result. Hinting vertical stems might solve some ot these “irritations“, but might cause strange spacing issues.