retargetlinks.js

// ==UserScript==
// @name         Links target current tab
// @namespace    https://tampermonkey.net/
// @version      0.1
// @description  Rewrite links to target current tab.
// @author       Tanath
// @match        https://*.tumblr.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    var x,i;
    x=document.links;
    for(i=0;i<x.length;++i) {
        x[i].target="_self";
    }
})();