While it won't become that bad ever again because of far more and better standardization, it has basically become a Webkit monopoly already. Sites often don't work (as well) on Firefox because web developers don't bother to do cross-browser testing anymore.

That was the actual (only) good thing with Internet Explorer: it coming with Windows endured significant adoption since many people don't bother installing another browser, especially in business environments. This forced web devs to make their sites and apps cross-browser compatible. With Edge being a Chromium browser that has gone out of the window.