Wednesday, April 24, 2013

Firefox preferences for screen readers

I realized that nobody yet shared that new user preferences in Firefox were introduced for screen readers so I do it here.

There are couple preferences to make screen readers support smoother. And all of them - you might be not surprised - are Adobe Flash related. Flash is headache for both assistive technology vendors and for Firefox developers because flash is tightly related with hangs, crashes and security problems. Unfortunately we need to balance between usability and stability when dealing with Flash so we provide a bunch of preferences to let the user make a choice.

No plugin sandboxing for JAWS

You probably heard or even encountered yourself that JAWS started to hang occasionally in Firefox 14. It was a regression from bug where OOPP (out of process plugins) option was forced on by default on Windows Vista and later. Since it was done on security proposes then we had to choose between making Firefox unstable for JAWS users and keeping JAWS working. So we let users to decide and introduced a preference


The preference is not public and you should add it manually via about:config. For that you need to type about:config into address bar and then choose 'New' -> 'Boolean' items from the context menu. As far as I know you don't need any actions on Flash side because Adobe turns sandboxing off as long as Firefox's one is off.

AFAIK JAWS fixed the issue on their side: they started to deal with flash content as separate document. It is definitely different user experience but it must be better than security hazard. So we have an open bug to remove this pref. If you have a reason why we shouldn't do this then please comment into the bug.

Note, the preference was introduced in Firefox 18 and it has an affect only if JAWS is running.

Delayed plugin creation

We had another report about hanging screen readers (Windows Eyes and JAWS) on web pages with Flash. To workaround the problem we introduced a new boolean preference:


This pref makes the browser to delay a plugin accessible creation. As a rule it saves from hangs. There's accompanying numeric preference used to specify a value of the delay in seconds:


By default the value is 10 seconds. This means you can't interact with Flash content during 10 secs after the plugin was loaded on the web page. It's definitely bad user experience but it's still an option to keep screen readers working.

You should be aware that user entered value of accessibility.delay_plugins preference is ignored as long as JAWS or Windows Eyes screen reader is detected. In this case a plugin accessible creation is delayed automatically.

This feature was introduced in Firefox 22.


  1. (1) Does Firefox have other (hidden - or not) preferences that are *just* related to users of accessibility features?

    (2) Could it be an idea to make longdesc support (in the user interface) depend on a preference?

    1. Hi, Leif.

      1) I don't recall. But preferences are introduced/removed in Firefox constantly. Some of them may squeeze unnoticed in front of me since I keep the focus on assistive technologies support.

      2) Mozilla has a practice of wrapping new features by preferences when new feature may affect on performance for example. In case of longdesc I don't see a need in a pref.

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