That is not a Firefox error message. It is a Windows message, administrators can set a quota for the amount of disk space a user can use.
If this is your own computer and you have administrator access, open Windows Explorer, right-click on the drive that Firefox is on (typically this will be the C drive) and select properties. Go to the Quota tab of the properties dialog and click on the "Show Quota Settings" button. Now de-select the option to "Enable quota management"