Are you on macOS Mojave? If yes, it could be because Firefox started honoring the system font settings of Mojave [1]. Enabling font smoothing in System Prefer

Are you on macOS Mojave? If yes, it could be because Firefox started honoring the system font settings of Mojave [1]. Enabling font smoothing in System Preferences now results in slighlty bolder fonts, and grayscale font smoothing. Firefox and Chrome needed some time to adjust to these settings, but now have the same rendering than the rest of the system. [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1495282 https://bugzilla.mozilla.org/show_bug.cgi?id=1495282