About 2 years ago I pointed out to Mozilla development that it was very difficult to find add-ons that were relevant to accessibility. The basic problem was that there were a limited number of categories and accessibility was not one of them.
The problem has not been fixed directly but two newish features for searching the add-on library have made it much easier.
Firstly, tags can be added to the description of an add-on and these include 'accessibility'. So you can get list of all add-ons with an accessibility tag. There is one wrinkle to this process, there does not appear to be any way of searching directly for a tag. The only way I have found to do it is to do a general search on 'accessibility', choose one of the add-ons and then on the right hand-side will be a list of tags, clicking on accessibility will give a list of all relevant add-ons. If anyone can provide me with a more elegant way of doing this I would be obliged.
Secondly any user can create a collection of interesting add-ons. These can be accessed by clicking on the 'add-on' menu and choosing 'collections'. You can then search for accessibility collections. Ken Saunders has produced one called 'Firefox Accessibility Add-ons ' which has an interesting list.
Between these two methods I can now find a reasonably comprehensive list of accessibility add-ons. If there are any add-ons that are not in these lists it is really up to the owners to tag them up properly.