Do you know which platform the visually impaired users are on?
Kathy and I experimented with Tor Browser 4.5a3 on Mac OS 10.9.5 and tried to use the system VoiceOver utility. But when the initial setup wizard is open, VoiceOver thinks that no windows are open in the Tor Browser application (and therefore VoiceOver does nothing – you cannot navigate among text labels, etc. because VoiceOver is completely unaware that the wizard window exists). This must have something to do with the fact that the browser has not completely started up yet (the setup wizard is modal and blocks startup in order to prevent the first browser window from opening).
Ignoring the initial setup problem, Mac OS's VoiceOver feature does work on Tor Launcher's Tor Network Settings window, although support seems to be somewhat buggy (e.g., you cannot navigate among radio buttons without appearing to choose them, and choosing them has no effect). So far, these bugs seem to be present in Firefox too.