I'm also having the same problem
Thanks to the two of you for posting about this problem. It allowed me to understand that it was not just me having the problem on just my one computer. Also, I surmised it was not affecting EVERYONE, because there would have been many more replies, so it helped me narrow the focus of my troubleshooting.
For clarity of the description of the problem, I have posted the solution that worked for me in a new discussion with the subject, "Clicking the LastPass Extension in Firefox Opens a Settings Pop-up Window That Won't Go Away (SOLUTION)". I hope it solves your issue.
The solution, in four words; refresh (not uninstall) Firefox.
@johnathandorneles
@tscharbo
The solution for me was to hide the taskbar on this Windows laptop.