Hi suego2, Firefox normally will open links in the same tab unless they are specially coded to open in a new window/tab. To open a specific link in a new tab, you can use one of these methods:

• right-click the link > Open Link in New Tab
• middle-click the link with the mouse wheel
• Ctrl+click the link

If you want links to open in a new tab with just a simple left click, you need an add-on for that. Here's one, for example:

https://addons.mozilla.org/firefox/addon/open-link-with-new-tab/

Make sure to visit its Options page to specify whether you want ALL links in a new tab or only links to other sites.

Note that Firefox may refuse to open links in the same tab in case this is a pinned tab.