Sites like Statcounter can track happenings on your Web site. For example, I can tell you of my last 500 visitors, 49% used Firefox, 27% used Internet Explorer, 6% used Google Chrome, 2% used Safari, 0.4% used Nutch, and 0.2% (one user) used Konqueror.
We see a lot of statistics regarding Web browser usage. It does vary by region. Firefox and Chrome seem to be more popular in Europe. According to Statowl, the latest numbers for the US, around 60% of users are running Internet Explorer, and only 20% running Firefox. World numbers are somewhere around 47% for Internet Explorer and 31% for Firefox, according to Statcounter.
None of these will be 100% accurate, but they do paint a picture for us. Why are my users more likely to use Firefox than the average world user? I would assume that, as I maintain a technology-related blog, my users are more likely than the average user to use a browser like Firefox.
If I were doing Web design and I had these numbers, I would need to make sure the browsers my users use were the browsers I was testing my site in. Though it really should not happen, there are times a page looks different in Internet Explorer and Firefox (for example).
It is interesting for me, as someone who is not running a for profit site, to look at these statistics. For someone who IS running a for-profit site, this information can be critical.