Whilst using FF20.0.1 and now with FF21 the new tab thumbnails fail to open for each new session. Why?

Question

For a long time my chosen websites appeared as thumbnail images on the new tab page when I began a new session, maybe since about FF15, I don't recall for sure. Recently, however, whilst using FF20.0.1 this behaviour stopped and none of the thumbnail images for my selected websites opened up, although the links to them remained and this behaviour has continued to FF21, which I have been using since yesterday (19th May). This is not a serious issue, but I do like things to work properly and to understand why they don't. I don't know why there should be a relationship between the two events, but the problem began after having opened the new tab page and before the images opened up I clicked on one of the links - where the thumbnail image would normally be - to go to the website; thereafter FF failed to retain a memory of the thumbnail images from one session to the next. By the way, I never empty my history cache only my main web cache that has previously had no effect on my new tab thumbnails, not since about FF15 or so anyway.

Answer 1

Cor-el, hi.
When I opened FF about ten minutes ago I noticed that the new tab thumbnail image I had brought up from my previous session had been remembered, so I opened some of the other websites listed to bring back their images too; I then shut FF down, re-opened it and the new tab page to find all the images had been recalled. Next, I shut down the computer to determine whether or not the images were getting lost that way; on rebooting the computer and launching FF I again found the images were still there. Now I closed the new tab and emptied the main cache (43 MB); on opening the new tab the images were still opening (you will recall that when the new tab thumbnails first came out they disappeared on emptying the main cache). Bearing all this in mind I have not yet tried your suggestion of temporarily disabling the add-ons in safe mode because if the thumbnail images came up that would not currently tell me anything. Also, the problem I described in my first post does not remotely coincide with the installation of any add-ons, which have not been recent. Given then that my thumbnails are appearing after the cache has been cleared where does FF get them from now? I still don't understand this recent behaviour and why for the moment the problem is not occurring. Thanks again.