вот еще скриншот

I looked at https://fonts.google.com/specimen/Open+Sans and when I compare Firefox and Chrome, Firefox seems to be using heavier strokes for light, regular, and bold characters. (Screen shot comparison attached.) This may be a change in ClearType (anti-aliasing) code or preferences in Firefox 56.

There are a lot of changes being made in Firefox right now. I searched to see if I could discover any recent changes regarding anti-aliasing but did not find anything specific.

To prevent Firefox from making letter "stems" wider, could you try this change:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste cleart and pause while the list is filtered

(3) Double-click the gfx.font_rendering.cleartype_params.enhanced_contrast preference and change the value to 0 ( by default, Firefox checks the Windows registry to see whether you have set a value using the ClearType tuner, otherwise it uses 50).

I don't know if you will notice any change. Possibly you will need to exit Firefox and start it up again.

jscher2000 Thank you! But the changes are not significant. The font is still thick . I return to FF 55.

gfx.font_rendering.cleartype_params.enhanced_contrast - losing their sharpness