Rated 4 out of 5 stars

While the problem of "Error parsing XML: Enttity not defined.." can easily be fixed...

<menuitem label="&filemessageschoosethis.label;" ...

is back reference to

<!ENTITY filemessageschoosethis.label "&renamefolderchoosethis.label;">

which was likely the workaround that was done when the filemessageschoosethis.label went away in a previous Thunderbird version? Well, renamefolderchoosethis.label is now gone, so either need to find something else (or define one....edit all the locale files in the add-on?)

Or for quick, I just changed the menuitem to be:

<menuitem label="choose this folder" ...

After I did this, the preferences dialog will appear....however mine has an empty accounts list, probably something else changed in the internals (or got fixed?)

Think I'll just try the other add-on mentioned in another review....until that review mentioned, I hadn't noticed that the Startup Folder functionality had stopped working, just that something was different and I didn't like it. It was adding a couple more accounts to my Thunderbird at work that led to discovering that I couldn't use this add-on anymore....