That may be because the default value for the following preference in about:config is set to 3. To have it open in the same tab, set it to 1 (see the answer linked below). browser.link.open_newwindow https://support.mozilla.org/en-US/questions/1328185#answer-1396449

Links can have different attributes which will direct a link to the current tab or a new tab. Clicking on a link in the Reddit Firefox sub opens it in the same tab on my browser. This add-on may help: https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus-webextension/

This add-on enables you to specifically open a link in the current tab. https://addons.mozilla.org/en-US/firefox/addon/stay-current/

Search sites often have a setting to let you choose between whether to open the link in a new window or not (Firefox diverts the new window to a new tab instead).

I don't see a setting like that on an eBay search results page, but I'm not logged in, so maybe it's an account setting?

If not, the preference TNorth mentioned will cause Firefox to ignore a site's instruction to open a link in a new tab. If you use that setting and you DO want a link to open in a new tab, you can use one of these methods:

• right-click > Open Link in New Tab
• middle-click the link with the mouse wheel
• Ctrl+click the link (tends to be a bit less reliable)