Add link to docs on Firefox's A11y inspector (reactjs#1525)

Author: Vinnl

content/docs/accessibility.md

@@ -480,6 +480,7 @@ to assistive technology, such as screen readers.
 
 In some browsers we can easily view the accessibility information for each element in the accessibility tree:
 
+- [Using the Accessibility Inspector in Firefox](https://developer.mozilla.org/en-US/docs/Tools/Accessibility_inspector)
 - [Activate the Accessibility Inspector in Chrome](https://gist.github.com/marcysutton/0a42f815878c159517a55e6652e3b23a)
 - [Using the Accessibility Inspector in OS X Safari](https://developer.apple.com/library/content/documentation/Accessibility/Conceptual/AccessibilityMacOSX/OSXAXTestingApps.html)