How to fix an upgraded Firefox profile

I'm an idiot. I ran a new version of Firefox nightly without creating a new profile. When I went to run my regular Firfox, I got this error message:

Here's how I fixed it.

1. Close down Firefox
2. Open a terminal and run:
3. firefox --allow-downgrade
4. Let Firefox start up and create a new profile. Then go to:
5. about:profiles
6. You'll see something like this:
   
7. Select "Set as default profile" and click through any warnings
8. Close Firefox.
9. Once again, run firefox --allow-downgrade

That should do it! Next time you start Firefox normally, everything should be fine. Probably.

In the future, make backups of your $HOME directory, and create a new profile for your Nightlies!