I have been trying for some time to get the bottom of why ⌥⌘\ will sometimes not bring up 1Password mini and on other occasions it will. I have at last understood that if I am browsing in a Firefox private window (as I normally do: see the Mozilla explanation of private browsing) that command will never bring up the 1Password 7 mini. It will do so when I am not in private browsing mode.
That is a regression from the 1Password 6 behaviour. With 1Password 6 I was always able to use that shortcut to bring up 1Password mini, even when browsing in a Firefox private window. In that case selecting a site in mini and pressing enter would open a new non-private window and log me in to that site. To my mind that was a much preferable workflow as I now always have to remember to open first a non-private window and only then use the shortcut to bring up 1Password mini.
I am using Firefox 60.0.1.
1Password Version: 1Password 7.0
Extension Version: 18.104.22.168
OS Version: macOS 10.13.4
Sync Type: WLAN