Add-ons I've used and tested and found to be very helpful especially for persons with visual impairments. These focus on a user's interactions with sites and Firefox and not on testing accessibility
A no-clutter image viewer for Firefox that allows you to zoom, rotate and display images/videos against a custom background. ImageTweak is a must-have addon for any photographer or photography-loving user.
Define a minimum size for all web text and toggle it on and off with a toolbar button. For Firefox 35.0 and newer. NEW: Three preset sizes. NEW: Scroll response for the toolbar button.
Mouseless Browsing (MLB) enables you to browse the web entirely with the keyboard. The basic principle is to add small boxes with unique ids behind every link and/or form element. You just have to type in the id to trigger the corresponding action i....
Simply resize/scale any images on the web by dragging an image!
Live demo: http://eliotvu.com/apps/image_resizer_demo
"Styles Tuner" allows you to change fonts and colors for all sites in your browser.
A multi-color highlighter for marking text on web pages. Uses SQLite to keep a record of your highlights on your system, AND NOWHERE ELSE. Created because Yooper doesn't support Firefox 4.0+
Large, colorful 64x64px toolbar buttons for Firefox.
Another RSVP reader, more focused on the presentation of the text, rather than many options.
Candy Tabs makes your Firefox browser a thing of beauty and elegance, differentiating each open tab with a color which helps distinguish your browsing so you always know which tab you want to select
Note: Not compatible with Firefox 25+ (try FindBar Tweak instead). In Firefox 24 and below, adds a find bar button that allows you to search for a word/phrase on a web page and lists the results. Also displays the total number of matches found.
HYPER-ANCHOR is the next generation of bookmark. You can make HYPER-ANCHOR for any element of a webpage (sentence, image, table, etc...), and put it into the bookmark menu. When you use HYPER-ANCHOR from the menu, you can easily return to the element...