Interestingly, the oldest archive of that addon page says version 4.14 was released three weeks after that XKCD strip - so excepting that it probably didn't have all the features of xmonad, we did have tiled browsing before XKCD suggested it.
Unfortunately, it's among the many killed off when legacy extensions were dropped. There is a webextensions version now, but it's not very useful - all it does is open and position multiple windows, instead of doing tiling within the same window.
