Two major accessibility features missing in Firefox: invert colors and greyscale. There are currently no good extensions for this. If someone can develop one, that would be amazing! This is what it should ideally include:
-invert colors and greyscale should independently toggle on/off through separate, customizaable keyboard shortcuts
-both invert colors and greyscale, when activated, should be global to firefox - when toggled on, it should apply to all pages, all tabs, and all new page loads
-both invert colors and greyscale should apply not just to page content, but also to the firefox theme (i.e. the toolbar, address bar, tabs, scroll bar, etc.). Invert colors in particular is useless if it only applies to the page content, because the firefox theme is too bright in contrast to a dark, inverted page (and the dark firefox theme cannot be used to fix this problem because it contrasts too darkly when the page is not inverted)
-invert colors and greyscale should both have independent checkbox options to exclude photos from the inversion or greyscale
These two items are the biggest display issues missing from firefox. If someone put them into a single extension, that would be a major service to the community.