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

Fonts are much beefier for me on OS X. Here is a side by side of Chrome looking at the same PR.

https://i.imgur.com/U5eQdqZ.png

https://i.imgur.com/wHKa4Zu.png

Edit: This made a difference for me, but it's also changed the feel of every UI element on my mac. I think it's a net win but will take getting used to: https://www.reddit.com/r/firefox/comments/aj3p2v/fonts_too_b...

tl;dr - disable 'use font smoothing when available' in system preferences. After killing and restarting Firefox everything looks normal.

yeah it is like everything bolded... not just in the menubar, but I think the rendering of webpages is affected too. I feel like I am on a Windows machine. ️