There are variety of fonts available for download and some of these have better readability compared to others. Many times readers find it difficult to read content from websites due to fancy fonts. Font Replacer is a add-on for Firefox which has the ability to replace a particular font with another one specified. The advantage of using this add-on is that if you find any website non readable, you can easily change it with your preferred one.
To replace fonts on Firefox, just type in the font you want to replace on the left hand side box and type in the font replacing it in the right hand box. Click save and next time you open any website with the font you have substituted, the fonts are changed when you open the web page.
You can configure the font settings from preference of this extension by opening the add-ons Manager.
The add-on is currently an experimental one, but works perfectly with most of the pages, although some pages might not load properly due to CSS issues.