It is configurable and it seems to work well for me for the time being. It indeed makes the text more readable (more than what the "Font Contrast" add-on does).

The 1 star down is because there is no way to set options per site (whitelist/blacklist etc) and also (as the developer says in User Guide):

1. The add-on is now abandonware so no more updates to keep up with changes in websites

2. The developer uses obscured javascript code to deter intellectual property theft and for this the add-on was going to be removed in 2019 from the official firefox add-on page. For some reason it is still available however.

I've just installed it an first impressions are very good.

Please add auto textwrap option

1. Please increase the line spacing because the lines are too crowded for larger font sizes. Let me change the line spacing as I like.

2. I would like the addon to remember its site's settings.

I find this extension extremely useful, but it always hangs on site https://msdn.microsoft.com/vba/office-vba-reference. For most pages in the site, I regularly get the message "A script in the extension "Legibility" is causing Firefox to slow down". The interruptions continue even after I uncheck "Enable" in the Legibility pop-up, and the only way to browse without disturbance is to disable the extension under about:addons. To re-create the problem, navigate to the above website then click random entries in the left-hand column under "Table of Contents". The extension misbehaves only on these Microsoft pages.

Works as expected, great addon. However I feel like there should be a option to make domain-specific settings, instead of whole web.

The addon is great, it has really helped me with my eye strain issues due to the increasingly popular design trend of low contrast and tiny text sizes. I don't have poor eyesight, but I like having a reasonable text size and contrast so I can actually read it without having to keep on re-reading it due to the uncertainty caused by it being hard to see.

There's only one major issue for me: When a neutral grey text is overlayed on top of a dark or black background, the addon only shifts the text to a darker colour like black - making it invisible! It would be good if the addon was aware of the parent element's background colour and to factor that in so that the text colour would shift more towards white rather than black when on a black background.

Great solution for all the web geniuses out there that think medium gray text on a light gray background looks slick.

I don't understand all the knobs, but setting
Text Contrast 0
Minimum 100
Maximum 100
works for me. Thank you!

When I first reviewed this add-on I thought it didn't work. That's because I was testing it on addons.mozilla.org. For some reason it doesn't work on that site, but it does seem to work on all the other sites I tried.

Text Legibility allows you to set various parameters globally; that is, the settings you choose will apply to all sites. This is convenient but it can lead to some inconsistent results. Firefox's built-in zoom feature enables you to tailor zoom levels to individual sites.