If the current profile is not default, external links will be opened in a new window using the default profile.

External links will not open in private windows, if there is no normal window, a new one will be created.

Check that the browser.link.open_newwindow settings in about:config have not been altered.

Normal Links:

• browser.link.open_newwindow
  1:current tab; 2:new window; 3:new tab;

JavaScript Links:

• browser.link.open_newwindow.restriction
  0:divert always; 1:divert never; 2:divert no features

External Links:

• browser.link.open_newwindow.override.external (see browser.link.open_newwindow)

I appreciate the responses, but I am not a computer expert and these responses do not tell me much about how to solve the problem.

I have been able to navigate to the advanced preferences page and located "browser.link.open_newwindow" which is currently set at 3. I tried switching it to 1 or 2 but neither of them solve the issue of Firefox opening a new window instead of using the one that is already open.

Also, I have noticed that this line on the advanced preferences page is in bold type while all the other lines are not in bold..... "browser.launcherProcess.enabled" .....TRUE. I don't know if that has anything to do with the issue

Visit about:profiles in the address bar and make sure the profile with the label "This is the profile in use and it cannot be deleted" has "Default profile: yes". If not, you can click the "Set as default profile" button.