Do you have this option set?

You can check this pref> on the about:config page.

External Links:

• browser.link.open_newwindow.override.external

If this pref has the default value -1 then browser.link.open_newwindow is used. The meaning of the value of browser.link.open_newwindow.override.external is the same as of browser.link.open_newwindow

• 1:current tab; 2:new window; 3:new tab;

jonzn4SUSE - I ticked "Open links in tabs instead of new window" and that worked for a while but has reverted back to opening a new window despite that box still being ticked.

cor-el - I cannot find about:config. Would you please point me in the right direction?

Thank you

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

When I open a link in a mail the link opens in a complete new Firefox session, not just in a new window. I used about:config. Changed browser.link.open_newwindow.override.external instead of -1 in 3. But that doesn't work. The link opens still in a new session of Firefox. In the task bar I see a new Firefox session. Normally when I open a new window in Firefox I see 1 Firefox session in the task bar and I can choose between 2 windows. What can I do about it?

By the way: "Open links in tabs instead of new window" is marked and it works fine within Firefox. Only external links have this problem.

That is my problem too - "Only external links have this problem."

When opening an attachment such as a PDF or spreadsheet for example, it opens in a new Firefox session window AND disables Firefox as my default browser.

I want to have only one Firefox browser open at any time, not 2 and I want Firefox to always be my default browser. I have configured these in Options - “Currently Firefox is your default browser” and “Open links in tabs instead of new windows” but both of these instructions are overridden.