Hi ddugger, Firefox takes its cue from the way the link was coded in the page. If the link is targeted to open in a new window, then Firefox normally will divert that to a new tab instead. If the link is not targeted, then it will open in the same tab. Is that the problem, or is the problem with external links you are opening from other applications?

Internal Link Issue

You can use an extension like https://addons.mozilla.org/firefox/addon/open-link-with-new-tab/ to control what Firefox does with untargeted links:

• same tab
• new tab if it's a different site
• new tab always

External Link Issue

Could you check these settings:

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

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

(3) Review/Edit the preference values as follows (double-click the current value to open an editing field, and then when you're done, press Enter or click the blue checkmark to save the change):

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

This is the one that has a checkbox on the Preferences page and determines whether targeted links are allowed to open in a new window or are diverted to a new tab instead; there's no effect on untargeted links:

• 3 = divert new window to a new tab (default)
• 2 = allow link to open a new window
• 1 = force new window into same tab

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

By default, if a page sets width, height, or toolbars for a popup, Firefox will open it in a "dialog-style" window. To force those into a tab as well, you can change this preference to 0, but that's not directly relevant to your question:

• 0 = apply the setting under (A) to ALL new windows (even script windows with limited features)
• 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

This preference governs external links that open in Firefox:

• -1 = apply the setting under (A) to external links (default)
• 3 = open external links in a new tab in the last active window <= You could try this if -1 isn't working
• 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, there's an arrow-like reset button at the right end of each of those rows if it has a non-default value.

Was that different than what you had?

Note that Firefox 78 is the last release for Mac OS X 10.9 and 10.10 and 10.11 and Firefox 78 users will be moved to Firefox ESR update channel (you get 78.0.x ESR), you will need at least Mac OS X 10.12 to be able to use next Firefox 79+ releases.

• https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac
• https://support.mozilla.org/en-US/kb/macos-users-esr