Edit: Dev was responsive to my comment, though I found the issue to be a browser setting problem. If you have similar issues with large amounts of data not saving properly, you can adjust the values dom.storage.default_quota and dom.storage.default_site_quota in about:config.

Original: Saving large values seems to cause problems. It fails to update the key and can cause a loss of all of your local storage if the large value is the first in line to be saved. I'm guessing there is an overzealous timeout delay or something. It is handy for viewing the full value of a local storage key if it exceeds what is shown in the inspector though.