JAWS and NVDA will continue to use IA2 in Firefox.
However, enabling native UIA in Gecko causes them both to try to use UIA, which causes breakage.
The same is presumably true for other Vispero products, as they use components shared with JAWS.
Future versions of these products will handle this correctly themselves.
Until then, disable UIA when these products are detected.
As part of this, the UIA pref has been changed from a bool to a uint32_t.
This will cause existing configurations of this pref to be reset to the default, but this is okay because this pref has never been documented, exposed in the UI or intended for public usage.
This allows for three values: 0 to never enable, 1 to always enable and 2 to enable unless incompatible clients are detected.
This makes it possible for developers to test those clients with Gecko's UIA implementation when necessary.
Differential Revision: https://phabricator.services.mozilla.com/D243245
ba45223d7c