Comment 2 for bug 1540129

I’m seeing that too on my devices, and I’ve been able to "fix" it by doing the following:

 1) open a new tab
 2) in the new tab view, switch to the top sites grid if needed, identify one top site that doesn’t have a thumbnail, and tap/click it
 3) open a new tab again, and go back to step #2 until all top sites have thumbnails

As a corollary, a possible explanation for the bug is that missing thumbnails were never generated in the first place.
A thumbnail is generated shortly before a tab becomes invisible, so typically switching between tabs or opening a new tab will generate a thumbnail for the tab that was current. On the other hand, closing the current tab or using the address bar to browse to a different website in an existing tab won’t generate a thumbnail.

My suspicion is that those top sites that are missing a thumbnail are typically being browsed to, and then either the tab is closed or the address bar is used to browse to a different site.

To fix that, we need to find the right moment to generate a thumbnail for the current tab, once it has finished rendering.