"open new windows in a new tab instead" has nothing to do with opening bookmarks. That is from webpages.

If you had bookmarks opening in a new tab with a "left-click" automatically, I suspect that you had an extension installed which provided that feature; said extension now being disabled as it may have been a Legacy extension that is no longer compatible.

Without the need for anything "extra" - both {Middle-click} or {Ctrl + Click} should open the bookmark in a new tab. Always works for me!

I use a trackball that has a wheel over the middle button. I really want to do it the way I always have... Left click the bookmark.

The browser should accommodate me. I don't want to waste valuable time relearning something that used to be automatic.

Well, I am happy to report the solution:

Open a new tab and go to about:config (remember that is a colon not a dot.)

search for browser.tabs.loadBookmarksInTabs

double click the line to change from "false" to "true"

No add on needed. Just a setting change.

This was found at: Firefox 57 gets a Preference to open bookmarks in a new tab

TomFitzp said

Well, I am happy to report the solution: Open a new tab and go to about:config (remember that is a colon not a dot.) search for browser.tabs.loadBookmarksInTabs double click the line to change from "false" to "true" No add on needed. Just a setting change. This was found at: Firefox 57 gets a Preference to open bookmarks in a new tab

Thanks for your solution. It's more useful than the chosen answer!