2 suggesting(main issues):
1) in Paul.Grenfell's review
increase "Capture Delay" and "Capture Timeout".
The default setting is unuseable, when I try to reflash many websites thumbnails at same time, they all become blank image.
2) set "defaultRefreshInterval" to 0
the default setting was 1day
Because when people first time use speeddial, they have no chance to know those 3 settings in about:config. Besides, issue 1+2 will cause too many thumbnails become blank image and slow the speed dial when opening it.