Firefox 68 introduced a new security measure to prevent untrusted local files from reading files in the same folder or subfolders. Unfortunately this also prevents StatPlanet Cloud from running on your local drive / offline.

The new security measure can be disabled as follows:

1. In Firefox, open up a new tab. Type or paste about:config in the address bar and press Enter.
    
2. A button appears asking if you accept the risk to change advanced settings. Click that you accept the risk.
    
3. Type security.file in the search box, which will filter the list for words containing this word.
    
4. Double-click the security.fileuri.strict_origin_policy preference to switch the value from true to false

Please note: There is a reason why Firefox 68 introduced this new security measure. It is important to avoid saving any pages or files from untrusted websites. If you are not sure that a site can be trusted, then make sure to save the files or pages in a new folder such as a sub-folder 'untrusted' in your downloads folder. This will prevent the file from accessing any valuable content within the folder it is saved.

If you have an older Firefox version, and do not have the above configuration option, please search instead for privacy.file_unique_origin and change it to false.