I found similar issues in the forum, but this issues seems to be slightly different. The fix offered in the other threads (upgrade to extension 3.9.19) does not resolve the below issue.
When using Go & Fill in Firefox, the website will open in a new tab and the login information will populate. In addition, a very tiny-sized new browser window will be launched, appearing as a grey circle in the upper left corner of screen and pull focus--no website is opened in this new window. (If I resize the new window, I can close it. If I click on the new window, it opens in full screen mode). A new tiny new browser window opens in this manner each time Go & Fill is used in Firefox. This issue seems to have emerged in the past 1-2 weeks. In that time, I don't believe I have upgraded the browser or 1Password. I upgraded to the latest stable release of the extension today, but the issue persists.