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
The extension generates a documentmap or index of any web document structured with headings and/or with sections in HTML 5. It shows the headings structure, the errors in the structure (ie. incorrect levels), and it works as HTML5 Outliner too.
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.
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.
Define a minimum size for all web text and toggle it on and off with a toolbar button. For Firefox 33.0 and newer. NEW: Three preset sizes instead of only one.
Large, colorful 64x64px toolbar buttons for Firefox.
Another RSVP reader, more focused on the presentation of the text, rather than many options.
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+
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.