Rated 3 out of 5 stars
I found the problem I was having with the add-on. I was seeing only a popup window with a list of my account names. The add-on wouldn't show or do anything else. No menu, no buttons, just the account list.
You can see what it looks like here: https://i.postimg.cc/VkWqM344/MSF-2.jpg
The reason: it doesn't like my desktop video config. It needs to be fixed for compatibility with HD desktops/displays.
The popup window is permanently stuck above the desktop frame (out of bounds) and there is no way to force it down once opened, IF I run the add-on with Tbird in my normal desktop mode (1920x1080).
I had tried changing desktop resolution before, but I didn't go far enough with my experiments. Turns out, there are two ways to fix this:
1.) Lower desktop vertical resolution to 1024 or below. The top bar, menu/tabs and buttons become visible. Then, drag the window down and the top portion of the window will stay in view when desktop resolution is increased to normal.
2.) This method may not work for everyone, depending on their graphics card/driver. Switch display via driver from 1080p to 1080i while the popup window is stuck out of bounds. That forces the top part of it into view. Changing from 1080i to 1080p with the window stuck out of bounds also works.
Lastly, the window must be maximized in order to scroll to the bottom of a long accounts list and to see the Local folders bar, Restart Thunderbird and Close buttons.
Kind of a pain to have to do this, but beggars can't be choosers.