Nifty plugin for making web page text more readable.
If anyone is interested in its Chrome version, here is link to the plugin's chrome port.
https://chrome.google.com/webstore/detail/font-contrast/ahmldabkdfgojihjidnfdbmigjcmkdpm
Chrome port works exactly as the Firefox plugin, except for toggle on/off button.

At a glance, works better than Google's native equivalent on Chrome. Do take a moment to read through the configuration options!

Gripes: As others have noted, the icon is (ironically) invisible on dark themes. They should re-do it to make it visible.

It's a bit complicated to set it functional, but it works, as a font-zoomer, that's it. Nice job for saving my eyes from those stupid micro-font webs, thanks.

Very useful addon. Also for people with Achromatopsia

If I select blacklist, it resets zoom level. So I adjust zoom level, and it deselects blacklist. Can't really change anything at all.

Works great if not using an extension to make websites darker. It doesn't really change the contrast of light text on dark backgrounds. (I played with all the options.) I don't think this is the fault of this extension though because the other extension takes over all styling. So no stars deducted.

I like the option to increase the text size too. It adds the missing toolbar icon that Firefox refuses to give us for toggling custom font size.

Text looks a lot better in pages with lighter backgrounds and black text. I also love all the great options for getting things just right, and I love that the changes can be customized for each website, or use blacklist mode.

doesn't work

I have macular degeneration and I thought I was going to have to give up on Firefox. This extension makes it possible for me to keep using Firefox.

This add-on basically applies a high-contrast theme to site fonts, getting rid of the boring gray text that so many sites are filled with (which I only realized when I started using this). It works perfectly and you can easily turn it off and on for different pages. You can also whitelist or blacklist sites so that sites you visit regularly look good with no fiddling required. There are plenty of settings to mess with though if you want. On default settings it's actually pretty smart about where it applies the contrast fix--it doesn't blacken links or text that's supposed to be colorful, and it has made everything easier to read for me so far. You might think it would look janky but actually, everything looks cooler when the text has some weight and doesn't blend in with the background. It's a subtle "digital" aesthetic. Perfect add-on.

Works well, except for sites which abuse the !important tag in their css. For example
the reviews on www.wilko.com. It's littered with !important tags. I was able to temporarilly make the pale grey text black by deleting one "!important" from a tag in the Firefox console, thus proving the point. Perhaps there's a way to override these evil tags in the addon?

Really helps on HIDPI screen that is somewhat washed-out. Good work!

Breaks Google Calendar on my Asus tablet using Firefox 89.0

The most useful extension I have ever used. Saves constant zooming in and out of web pages to read small faint fonts. Wonderfully customisabe, easy to use and light on resources. Many thanks to very capable dev.

It's challenging reading grey font on a white background, and this is the best solution I've found so far. Looking forward to testing it out thoroughly. Curiously, the only page it hasn't worked on so far is the mozilla add ons page (https://addons.mozilla.org/en-US/firefox/addon/font-contrast-fix). Perhaps I'm doing something wrong?