Obviously, but that is a self-reinforcing loop. I'm not suggesting that government websites drive the most traffic or anything, but the government is kind of special as an entity. In several other areas the US government is bound to show no preferential treatment to vendors or other entities, such as in public broadcast TV or awarding government contracts. I don't think "internet browsing software" is one such covered area, but forcing people to use one browser to access their websites is pretty equivalent in this day and age, so if they drop support for Firefox a lawsuit might change that.

My point with the money is that a whole team of highly skilled QA professionals isn't even a rounding error on that kind of balance sheet, but thinking about it further there's a solid argument to be made that supporting a variety of web browsers for government web services is in the interest of national security. In that case they could pull the money from the military budget for the project.