[orca-list] Flat review issues in Gecko aplications

[Alex ARNAUD <alexarnaud member fsf org>]

I've noticed several issues with flat review in Gecko applications.

Test environment :
- GNOME Shell 3.18.3
- Orca 3.19.91pre (updated from master)
- Firefox 44.0.2
- Thunderbird 38.6.0

Steps to reproduce in Firefox :
1) Open Firefox on a wikipedia page, meybe that : 
https://en.wikipedia.org/wiki/Flower
2) Press orca+i to read current flat review line, it works
3) Press CTRL+Down to go to last line of the web page
5) Press orca+i to read current flat review line, it fails and reads the 
tab bar
6) Press "page down" to display last line content
7) Press orca+i to read current flat review line, it fails again

Steps to reproduce in Thunderbird :
1) Open Thunderbird
2) Press F6 that jump me to "inbox" item
3) Press orca+i to read current flat review line, it fails (read nothing)
4) Press down arrow and retry to read current line, it works

Best regards.

--
Alex ARNAUD