Firefox has a font rendering quirk, where smaller fonts tend to not render very well. This is most evident on sites like Reddit, for me anyway. Luckily there is a fix. Open about:config and add/modify these settings:

gfx.font_rendering.cleartype_params.gamma 1500

gfx.font_rendering.cleartype_params.pixel_structure 5

gfx.font_rendering.cleartype_params.rendering_mode 5

Back to Firefox (for now)

I've used Zen Browser for a while (since the first public alpha), but it hasn't been smooth sailing lately, mostly due to some weird bookmark bar bugs. I feel like originally it was just a better looking Firefox, but lately it seems to be going a direction that adds a lot of functionality that I don't really use or need. It's nice, and even exciting, to have new features, but not so much if I'm having to work around them more than use them. I decided recently to just go back to Firefox, but I still find myself opening Zen from habit. This particular font issue has always bothered me in Firefox, so I did some research and found the settings about. It does look as well as Zen, but it's definitely an improvement over Firefox's default.