HTTPS is also cached by every major browser, meaning the image in question would be in your cache just as well as the screenshot containing it.
> Its silly because they turned off viewing the HTTPS websites on the 'newtabs' and replaced it with grey screens - yet they still take the screenshot.
If this is true you should file a bug for it. Bug 754608 / Bug 627239 / Bug 822867 suggests that this is not the case, and that capturing them was disabled entirely. In fact, those three bugs suggest that this bug is only open because a better replacement for grey-squares is needed?
> The sec-low status was given because they mention the low res screenshots are not enough to get anything useful - but I've just proven that not true.
This does not seem to match what I see on the thread. The bug was filed by a mozilla developer and the consensus seems to be that it is definitely an issue:
c0 - Mozillian files the bug
c4 - Mozillian mentioning they think screenshots of any size could be sensitive -- the opposite of "not enough to get anything useful" -- but also that it's not something that can be reasonably fixed.
c5 - Mozillian acknowledging its a problem and suggesting its difficult to properly fix
c8 - Mozillian noting this is an even bigger issue on OS X
c9 - Mozillian noting that the whole concept might need rethinking
c11 - Mozilla security manager assigns security classification to bug