Trying to limit this to Fonts, Zooming, Reading tools (also see collection For Visual Appearance for Backgrounds, Colours, and Themes / Personas, special effects, etc)
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
Shows full image when you hover over a thumbnail. Works with Amazon, Baidu Images, Bing Images, Facebook, Flickr, Google+, Google Images, IMDb, LinkedIn, Netflix, Pinterest, Reddit, Twitter, Yandex, YouTube, Wikipedia, Yahoo Images, & many more.
Theme Font & Size Changer lets you change the global font size and font family used for the user interface in Firefox. Design your browser with your favorite font.
NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.
Full featured zoom manager with independent page and image zoom modes. Choose from four zoom modes: Default Zoom, Fit-To-Width, Default Zoom + Site Specific or Fit-To-Width + Site Specific. Button style and zoom levels are fully configurable.
Set Default FullZoom Level,
Toolbar buttons for Page zoom and Text zoom.
Scrolls the page smoothly when scrolling the mouse wheel (or keyboard arrows if enabled).
Shows a full size image when you hover over thumbnails on popular websites like Facebook, Twitter or Amazon.
Does everything for an HTML table: Copy (table/row/column/cell), Sort (multi-column), Search (regex, range operations ...), Filter, Chart, Statistics, Rearrange, Combine/Compare. To use: Right click in a real HTML table for the "tabletools2" option.
Make your photos smaller, so they don't take forever to email or send to websites.
The Ultimate FaceBook Enhancer! Instantly add many tweaks to Facebook.com including: Large Images on Hover, Emotiocons, Select All Friends, Styles, and much more. A must have for Facebook users.