[orca-list] When checking "only speak displayed text" Orca speaks element context like "heading" and "link" on Firefox

[Alex ARNAUD <aarnaud hypra fr>]

Hello,

Environment:
- Orca 3.34 and master
- Debian 9 and 10

Steps to reproduce:
1) Check "only speak displayed text" in Orca preferences
2) Open Firefox and make a search on Firefox
3) Go over headings and links with the keyboard

Result:
Orca announces "X heading" or "X link".

Expected result:
I assume Orca shouldn't announce "heading" and "link" with "only speak 
displayed text" is selected.

Best regards,
Alex.