Hey, I also noticed font changes, I didn't see when FF updated, but in control panel I think it's the same day I noticed the font change? (I didn't memorize) I downgrade and it's back to normal, this new thin font is upsetting and harder to read - I uninstalled Brave browser because it was like this for me and it'd be sad if it stays like this :(

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting for changes to take effect.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

For me disabling acceleration did not help (recommended was already unchecked), neither did starting in safe mode, and my graphics card is old, so no new updates.

Disabling hardware accelerion doesn't help me also. And I've got old graphic card too (Nvidia 410M) with 388.71 driver installed.

Firefox gets worse with almost every version. I have reported a problem with cropped extension icons in march and it is still not fixed. 9 months... Also this problem with fonts and another one bug with scrolling that I reported today. There will be a day soon when I forget about Firefox after almost 15 years of using it and switch to Chromium Edge or Chrome.