I have used Firefox in Sandbox (for security). Until Firefox 56 there were no problems. But in Firefox 56.01 (open in Sandboxie) are experiencing problems. Firefox 56.01 no longer saves or updates my Email accounts and their saved passwords anymore in Bookmarks. That is, to enter my Email accounts, (in firefox 56.01) I always have to enter my email and password. In Firefox 56 I automatically login to accounts because unlike version 56.01, version 56 still saves all data (email accounts and passwords) on it. Then I decided do not update to 56.01.
We don't support use with Sandboxie. As you're seeing, it can cause unexpected behavior in apps. The app works perfectly fine on regular Windows, but when you run it within Sandboxie, some things can get messed up. You'll need Sandboxie to fix whateveres up with Firefox 56.
Sometimes, the impossible can become possible, if you're awesome!