There is a section of the following article about starting Firefox from a command line (you can use the Run dialog -- Windows key + R) in a manner that allows using an upgraded profile in an older version of Firefox. This isn't guaranteed to work if there were data file format changes in the beta version. But I think it's worth a try. See: [[Dedicated profiles per Firefox installation]] -- the "What happens to my profile if I downgrade to a previous version of Firefox?" section