This is in the editor, I believe, which I don't think would care what browser you were using.
The beauty of JSON files, as opposed to the rvdata2 files of Ace, is that they're human-readable. So you might be able to open them up and figure out why the editor is rejecting them. If it's only a small issue, you could potentially fix it yourself (use something like Notepad++ that won't add its own control codes - it needs to stay a plain text file). However, if there are large chunks missing or corrupted, that's a different matter altogether.
You didn't answer the first question about what happened last time you used it - did your machine crash or reboot? Did the editor crash? Did you close or shut down without saving your work?