Just tried on OSX with Firefox v47 and was able to restore a backup from April with no issues. I then upgraded to v48 and using the same backup file I get the "invalid format" error.
From what I can tell from the console errors it seems this Firefox update has broken the crypto libraries, so it will no longer support the wallet period. Those libraries are outside of my control and no longer actively maintained afaik, the last update was in January.
If you insist on using Firefox you will have to revert to v47.