Thumbnails in about:newtab don't show up with the disk cache disabled, a lot of people run around with it disabled and opt for browser.cache.memory.enable only for both privacy and performance.
Now, it's normal to lose these thumbnails due to the volatile nature of system memory when the application closes or when the OS is restarted, but the thumbnails aren't even saved in the session, no matter how many times the site is visited, essentially, memory cache users are left without thumbnails.
This also applies to when the disk cache is wiped clean, the thumbnails are lost and visiting the site is required for getting them back.
You already use storage for different parts of the browser that are exempt from the cache(history,bookmarks,cookies, etc), why not move the thumbnails to its own storage for this?
*** Bug 722160 has been marked as a duplicate of this bug. ***
My disk cache is enabled, but I got 2-3 thumbnails at best - the other tabs show no thumbnail.
(In reply to Ilia Penchev from comment #2)
> My disk cache is enabled, but I got 2-3 thumbnails at best - the other tabs
> show no thumbnail.
The thumbs sometimes don't generate after one visit to the site, I have noticed this as well, I could reproduce this on sites with a large amount of flash or java applets embedded, or sites with a large amount of hyperlinks before, not anymore though.
Will be fixed by bug 744388.
Fixed by bug 744388.