[Experimenter_1]

Can someone say what are the files in c:\users\[mainusername]\appdata\roaming\mozilla\firefox\[latestfirefox installation]\storage\default ?

It looks to me like they are supposed to be some sort of hidden user cookies.

 

[vanukuru.vinod]

They are installation backup files for Firefox, which are generally created by Firefox themselves as and when Firefox updates itself.