1) Because the perimeter of a circle grows as a function of the radius, an arc must grow just to represent the same share. This is misleading, giving an impression of growth where there is none.
2) It is difficult to tell which arcs on one side of the circle go with arcs on other parts of the circle. There is simply too much distance between them to judge accurately which arcs belong to the same time period.
But there is growth. Perhaps it's the static percentage based graph that's misleading, I find this graph of more or less the same data rather intriguing:
Your link is much easier to read.
However, that's not reflected in the visualization, and we're lead to believe that this is sampled with even steps over time.
Since the market has grown basically linearly, the arcs may actually be in direct proportion to the actual number of users, in which case there is very real growth.
The confusion comes when you see growth of absolute terms when the more interesting statistic is what the relative growth is.
Particularly, if one browser's arc remained the same size on each ring you might assume that it is staying the same. It has the same number of users, but it is losing prominence.
Note that this is from w3schools.com, not w3.org (a completely different site). This is for a web developer audience, so it kind of makes sense; from a developer point of view there are much bigger differences in standards compliance between IE versions than between Firefox versions. But mostly they just seem to want to keep a small number of columns on that page.
Also an nice reminder of how fast Mozilla/Firefox grew initially as well - I thought Chrome was exploding on the scene and had forgotten how fast FF took a decent market share. A neat reminder not to wear rose tinted specs.
But I found a 404 in the examples page:
It doesn't really help firefox that many of us geeks are switching to Chrome|Chromium|Iron for various reasons, too.
the empty space, which i believe is "other browsers" grow a lot then.