Changing the following setting in about:config to 'false' may achieve that: browser.search.openintab I say 'may' because 'false' is the default value. Otherwise there is an add-on that allows you to do that: https://addons.mozilla.org/en-US/firefox/addon/stay-current/ You can also right-click and select "Open link in new tab".

The add-on works if I right click the link and select "open link in current tab", but it looks like the default is still opening it in a new one. Is there a way to change that?

Did you look at the setting in about:config?

Yeah that was already set to false, switching it didn't do anything either.

For Google you can set this in the Google search settings (Where results open). Other search engines may also have a similar setting. Firefox can divert the new window to a tab via Options/Preferences.

For links that specify a target window there are prefs on the about:config page.

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

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