Could you describe the setting you changed? I think maybe you are referring to the following group of preferences. Check (3)(A) and (3)(C) in particular:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste neww and pause while the list is filtered

(3) Check your preference values:

(A) browser.link.open_newwindow - for links in Firefox tabs

• 3 = divert new window to a new tab (default)
• 2 = allow link to open a new window
• 1 = force new window into same tab <= Is this what you set to prevent new tabs?

(B) browser.link.open_newwindow.restriction - for links in Firefox tabs

• 0 = apply the setting under (A) to ALL new windows (even script windows)
• 2 = apply the setting under (A) to normal windows, but NOT to script windows with features (default)
• 1 = override the setting under (A) and always use new windows

(C) browser.link.open_newwindow.override.external - for links coming from other programs

• -1 = apply the setting under (A) to external links (default)
• 1 = open external links in the last active tab replacing the current page
• 2 = open external links in a new window
• 3 = open external links in a new tab in the last active window <= With (A) set to 1, try this one

To return to normal if things go haywire, right-click each "modified" preference and choose Reset.

Does that help with the problem links?

Thank you very much.

Apparently browser.link.open_newwindow.override.external was reset to -1.

I changed to 3 and everything is back to normal.

Thanks for reporting back. Not sure how one of the two preferences got reset and not the other one. Strange!