I'm sorry for the difficulty. We have had to make a few changes in response to Firefox 57. You can however turn previewing on with a preference. If you get getting a message stating you changed personas/theme, can you take a screen shot and send it? There was an issue in 5.0.2 that we attempted to solve on 5.0.3 but it may be that we missed a language translation. Thanks!
Also, this problem shows up when using Personas Plus, see: https://github.com/drsjb80/personaswitcher/issues/34. Make sure you have version 2.0.0 of that and see if you still have the problem.