Sounds like somethings not quite happy there dilf.
Was going to suggest mozbackup but may lead to the same probs again on a fresh install.
Have you added any recent extensions that may be conflicting with stumble? Prob not.
Personally what i'd do is d/l firefox but don't install it.
Create a new folder somewhere called 'fox bookmarks' or something similar.
Click bookmarks on menu bar.
Then orgnise bookmarks.
Click on file and then export.
Select the folder you created and save there.
I do this every week or so after i lost my fox alltogether and spent many weeks trying to get every bookmark back and in order (saving Bkms that is)
Time for an uninstall and a fresh install from the fox you just d/ld and 10 mins customising, 10 mins getting you're themes d/ld and installed, follow above steps and instead of exporting bkms just import from the folder you created and install the stumble toolbar.
Then reinstall youre fave extensions one by one and keep checking all is working ok.
When everythings as you want, d/l mozbackup and save all you're setting.
Very handy if you have any future probs or you want to set fox up on a laptop or another machine.
Hope this helps.
Colin.