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It shows how popular it is in comparison to other software programs. It isn't a gague of users - a lot of people have said they have downloaded it multiple times - but it is still important. It is also important to note that many users who have downloaded Firefox aren't counted in those statistics as they have downloaded it from other places - this is probably the most common method of distribution on Linux as most people use the built in package management tools to get them from their distribution provider and not Mozilla. It goes in both ways, but the number of additional downloads would be about equal across all programs so it is still important.