Rated 4 out of 5 stars

@Some Random Dude

You can "restart" the process. Go to:
Bookmarks (pull down menu) > organize... > organize (pull down menu) > create previews.

You can also change the settings to make a preview whenever you visit a page, so you could just open all bookmarks at once and previews will automatically be made for you...Also, in the profile folder there is a bookmark preview folder that contains the images, just delete all and begin the process again if you want.
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My biggest criticism is when organizing the bookmarks, in the thumbnails view, I will delete an image, and then the screen refreshes and then I am back at the beginning of the lot...I hate that. The problem is when I will be deleting a specific few BM's out a few hundred. After each deletion the screen refreshes and I am back at the beginning to scroll down again, each time...Please look into just removing the image, and not refreshing the screen upon deletion.

Other than that it is an excellent add-on.