The cure (temporary at least) for this error is to uncheck ,,Compact Databases" in Applications (Firefox/Mozilla). I don't know what is it but I think I don't need it anyway when I use CCleaner... ;-P
I had exactly the same error yesterday:
It appear when wanted to clean system & after this ccleaner crashed...
I was in luck cause I had system backup made 2 days earlier in Acronis :-)
But I wonder why this fault appeared and what kind of program made it...?
Please help.