Firefox 76.0.1 (default browser)
When selecting Go or Open & Fill from the MenuBar drop down it is not firing up Firefox (default browser) from the Dock, whether from cold or warm start and going to site and filling in credentials.
Firefox must already be open and on screen, then 1Password will open new tab and automatic credentials are filled in.
1Password Version: 7.5 (70500003)
Extension Version: 184.108.40.206
OS Version: macOS10.15.4
Sync Type: iCloud