I love when I click a bookmark that it opens to new tab because I don't want the open one to be overridden. Otherwise, I'll have to frequently right click on the bookmark or open a tab then the book mark.
There is a hidden setting for this. It would be great if it were made easier to discover and set. If you want to test it and see whether it does what you're looking for, here's how:
(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
More info on about:config:Configuration Editor for Firefox. Please keep in mind that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.
(B) In the search box in the page, type or paste browser.tabs.loadBookmarksInTabs and pause while the list is filtered
(C) Double-click the preference to switch the value from false to true