Use value of `aria-label*` to provide accessible name for tables in Firefox (#16588)
Fixes #5183
Summary of the issue:
In Firefox, tables labelled with aria-label or aria-labelledby (where the referenced element is not visible) do not have a virtual buffer node inserted with the accessible name of the table.
Description of user facing changes
Tables now have this node inserted in Firefox, as they already do in Chrome.
Description of development approach
Since Firefox returns the empty string if no description is given (rather than null as returned by Chrome), update gecko_ia2.cpp to check for the empty string as well as null.