[orca-list] Mouse review no longer works on Firefox

[Alex ARNAUD <alexarnaud hypra fr>]

Hello,

Tested environment:
- Debian 9 "Stretch"
- Orca master
- Firefox ESR 60 e10s disabled

Steps to reproduce:
1) Enable mouse review
2) Open Firefox on this page: 
https://en.wikipedia.org/wiki/Orca_(assistive_technology)
3) Try to read the first paragraph and after the second one

Result:
Orca stays silent most of the time, sometime I head "menu", sometime it 
seems it reads a link

Expected result:
Orca should read the paragraph when I move my mouse on

Best regards,
Alex.