Accessibility.tabfocus applies to xul
From MozillaZine Knowledge Base
As with web pages, pressing [Tab] when a XUL document is in focus will shift the focus from element to element. This preference determines whether or not the rules defined in accessibility.tabfocus also apply to XUL documents’ contents.
The main reason this preference exists is to honor OS X’s “full keyboard access” setting for XUL documents.
Possible values and their effects
Allows XUL elements to gain focus from [Tab] according to accessibility.tabfocus. (The default in OS X.)
accessibility.tabfocus has no bearing on focus navigation in XUL documents. (Default everywhere else.)
First checked in
Has an effect in