When I open a new tab, the about:newtab page is shown with nine boxes referring to recently viewed pages. I had the problem that these boxes were always empty, i.e. no preview was shown. As this worked with a fresh profile, I used try-and-fail to discover the preference setting that was responsible for that. It turned out that the effect was caused by the Preferences -> Privacy tab -> Clear History when Nightly closes -> Settings... -> History/Cache is checked ("privacy.clearOnShutdown.cache" preference). As I learned that the preview images are stored in the cache, it is reasonable that on the next start the images are no longer available, although this dependency will likely not be clear to a normal Firefox user. Nevertheless, I do not understand that for sites that I have visited in the current session, these preview images are not generated and/or stored and therefore are not displayed if I open a new tab later on in the same session. I consider this as a bug.

Duplicate or related to Bug 724408

(In reply to Klaus Malorny from comment #0) > As I learned that the preview images are stored in the cache, it is > reasonable that on the next start the images are no longer available, > although this dependency will likely not be clear to a normal Firefox user. Yes, this is bug 724408 (In reply to Klaus Malorny from comment #0) > > Nevertheless, I do not understand that for sites that I have visited in the > current session, these preview images are not generated and/or stored and > therefore are not displayed if I open a new tab later on in the same > session. I consider this as a bug. I remember this being discussed somewhere(email) and explained as expected (slowness when fetching thumbnails multiple times). I don't think there's an issue for this available anywhere though, so if you won't mind, I'll modify the title to reflect the second part of your post.

Will be fixed by bug 744388.

Fixed by bug 744388.