I don’t know the history of column span but the reason Firefox was “behind” on standards was because Google was pushing new standards through committee faster than competing browsers could keep up. Google would implement a new feature, offer it as a free standard, then get it through the committee. Because Google already had it in their browser, they were already compliant while Firefox had to scramble.

It was Google doing their variation of “embrace, extend, extinguish”

It got so bad that not even Microsoft had the resources to keep up. They said as much when they said they were adopting Chromium as their engine.