On: Solved By Flexbox – Cleaner, hack-free CSS

[jonahx]

I will help you, despite your snarkiness :)

Just use greasemonkey or tampermonkey and a script like this:

    var a = document.getElementsByTagName("a"); 
    for (i=0;i<a.length;i++) { 
        if (a[i].target="_blank") { 
            a[i].target="_self" 
        }                        
    }

It should work for the majority of cases. You should be able to tweak it to handle any exceptions.