Could you do a little poking around in Firefox's settings:

(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) Adjust the preferences as follows:

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

• 3 = divert new window to a new tab (default) <= Try this one (right-click > Reset to restore the default)
• 2 = allow link to open a new window
• 1 = force new window into same tab

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

• 0 = apply the setting under (A) to ALL new windows (even script windows) <= Try this one
• 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 in other programs

• -1 = apply the setting under (A) to external links (default) (right-click > Reset to restore the default)
• 3 = open external links in a new tab in the last active window <= Try this one
• 2 = open external links in a new window
• 1 = open external links in the last active tab replacing the current page

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

Does it work?

Yes, it worked. Thanks a lot. Hopefully it won't be a temporary solution!

I think it will stick unless you use the Refresh feature.

And once you know how, it's pretty quick.