Was it done because Mozilla thought that Chrome's increasing version numbers would make ordinary people think that Chrome was ahead of Firefox ?
But… the 3.6 -> 4.0 release was just such a pain. We spent more than a year trying to cram too many features in v4.
Clearly feature-based releases were just not cutting it, especially as Chrome was shipping new versions fast.
For the longest time people were saying it was the wrong move, but really - that was absolutely necessary, and proved out to be the right thing to do. The Stable / Beta / Alpha ("aurora", was that the name?) channels massively improved our yield :)
I detested Firefox from v4 onwards and finally got off the crazytrain at 12.
Other than the version numbering becoming vapid, the browser itself became vapid. No longer was Firefox about the users, it was about Mozilla.
That has remained the case to this day, and I have not bought a ticket to this day. I'm not counting the first ticket as a purchase since it was forced on me.
I'm hopping between the Pale Moon space station and (begrudgingly) the Chromium train now depending on what I'm doing.
If I remember right, after Firefox 4 they skipped a few versions and started the train model, while also trying to de-emphasize the version number in marketing.