Via email, I got this question:
I installed Office 2011 last night. I had Safari and Firefox open, but was forced to quit them during the Office install. Why?
During the installation of Office:Mac, we install fonts. Web browsers, since they’re heavily reliant on fonts, don’t particularly like it when you change fonts out from under them while they’re running. If you install a font that the browser is using, then it’s likely to not display that font properly until you restart your browser. To avoid this kind of issue, we simply ask you to close down your browser for a couple of minutes.
On a related note, Jim asked this in the comments thread in my post about latest Office 2008 and 2004 updates:
Why does the update to Office insist that I quit Safari????
Our updater uses some of the same code as our installer, and that’s one of the pieces we reuse. We reuse that particular piece of code because an update could include font updates.