[imp] Firefox 47 basic mode: next message marked Seen

[Oscar del Rio]

One of our users noticed that, in Basic Mode, after opening one email message, 
the *next message* is marked as Seen as well.
It seems to happen only with Firefox 47.0 and only in Basic Mode (he started 
noticing the issue recently, I assume when Firefox got auto-updated).

Older versions of Firefox and other web browsers behave normally.

The obvious solution is to switch to Dynamic mode or use another browser, but 
I'm very curious at what could have changed in Firefox 47 to cause this issue.
Any ideas?

Thanks.