The only way I've been able to reproduce it so far is by click the multi-quote button. It seems that Firefox is doing something it's not supposed to be doing, when you interact with the page in any way which fires off a background connection to the server, Firefox records each of those connections as a "back" page. A click like that can actually result in several background connections, and it fills up the back list.
From looking over their bug reports, this has been going on and off since at least mid-2016, on a variety of sites (Fox News, Stackexchange, Breitbart and many others. Firefox support keeps asking them for a list of plugins, blaming it on that, but users with unrelated plugs are having the same problem. I cannot get it to happen with Chrome, IE, Edge or Opera browsers. Apparently it doesn't happen in the Linux version of Firefox either, only Windows.
Do you guys/gals know which plugins you have installed on your browser? There's got to be a common denominator here.