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.