Text Mode -Good for those who are visually sensitive. loads pages in black and white/all text/obscures images. It’s also helpful for those who use screen readers. (Thanks to http://daigonite.tumblr.com/)
FlashControl -Stops Flash animations running by themselves. (Thanks to lysikan.tumblr.com)
BeeLine Reader -Uses a colour gradient that guides your eyes for those with difficulty reading monocolour text. (Thanks to nihilmeliusnihilpeius.tumblr.com)
Hacker Vision -Makes backgrounds black and text white to reduce the level of light your eyes are exposed to.
nyt_mod -Dims your browser screen-5 customisable levels (thanks to queermisandrist.tumblr.com)
F.lux/G.lux -makes the colour of your computer’s display change according to time of day (warm at night and bright during the day). Cuts down on harsh contrast. (Thanks to kelpforestdweller and anon)
Readability -Ensures every webpage is in a clear readable view with the option of disabling the page’s background noise and clutter. Great for the inattentive.
Hide GIFs -Turn on to hide any gifs from displaying.
Firefox: (Massive thanks to kiyoteruhiyama.tumblr.com for all these)
Image Block - Turn off all images on a webpage by clicking the button in the upper right section of the browser window.
Flashblock - Block all flash animations. (note that java must be enabled for this to work.)
Easy Read - Links, instead of being different colors, are now the same font/color as the rest of the text.
No Color - Disable background colors or images on websites.
Black background and white text - Self-explanatory. Can reduce eye strain or help those with sensitive vision.
Screen Dimmer - Dim the browser screen to reduce eye strain