Do you see empty thumbnails or they have favicons there? Are they associated 
with any urls at all?

Do you have any history? (if you have just cleared out your history it won't 
show anything - however if you navigated to addons site you should have at 
least that site in history)

they're just blanks with no url what so ever. been browsing for a couple of 
weeks without clearing anything

Could you please send me the list of other addons you have installed? it's 
possibly a clash between some of them. Or, if you have time you could disable 
all other addons and try only Jumpstart, if it works with all other disabled 
then it is definitely some kind of incompatibility with other addons.
mlalevic [at] gmail [dot] com

Seems like the problem was the extension "TooManyTabs". Seems to work without 
it.

Thanks for checking that for me, I renamed the issue to: "Conflict with 
TooManyTabs extension"
I'll look at it as soon as possible.

Fixed the issue - reminiscent of FF 3.0 code unfortunately.
Anyway, there's a workaround:
Uninstall Too Many Tabs, restart Firefox, install it again.
Basically if Too Many Tabs is loaded before JumpStart it would cause JumpStart 
to fail. Seems that extensions are loaded in order they are installed so if you 
uninstall Too Many Tabs and install it again it will be loaded after JumpStart.

I tested with 1.3.2 version of Too Many Tabs and data saved in it's tabs was 
unaffected by reinstall, however it would be sensible to backup data somehow if 
possible.

Fix is coming in next few days - I'd say before the end of the week.