Enter about:config in the URL bar and set browser.tabs.loadBookmarksInTabs = true.

Thank you. I need to try that out for a while before I am sure the problem is completely solved. I wonder why that setting was changed when I upgraded from 56.2 to 60+.

Hi Purebeads, I think this was a new preference in Firefox 57, so you wouldn't have had it enabled before.

Firefox 56 could run the legacy extension Tab Mix Plus, so perhaps you were using that to send bookmarks to a new tab. Depending on which of its many settings you used, there may be some other preferences you'll want to know about as well. Feel free to mention anything that needs to be changed and we'll see whether we know of a way to do it.

You helped me before to put tab rows in my browser, although they look clunky. I still want to move the tab rows down above the browser window, and move the bookmarks higher up.

Yes, I used Tab Mix Plus. Because of the lost customizability with the 60+ versions, and the poor rendition of many pages, I now use Opera and Chrome for most of my critical pages.

Purebeads said

You helped me before to put tab rows in my browser, although they look clunky. I still want to move the tab rows down above the browser window, and move the bookmarks higher up.

Unfortunately, I don't know how to combine multi-row tabs + tabs on bottom. I think I've seen it discussed over here:

https://www.reddit.com/r/FirefoxCSS/