There is always an unknown when you deploy new software to hundreds of people. All of your testing never simulates their techniques and a full load. But yesterday the cutover to 64bit Firefox 4 on the new server went beautifully. We announced the upgrade and at noon I just killall'd their processes on the old 32bit server and moved the binary out of the way to halt it from running. I then tar'd up all of their .mozilla directories, and extracted them on the new 64bit replacement server. I changed the global launch scripts and was done. The total process took about 15 minutes.
The shots below are the 'top' windows with about 65 users with Firefox sessions running. Watching the load split on the threads is a thing of beauty and scrolling is as crisp as it was with just our beta testers. There are a few spikes in the 5-8% busy area, but the bulk of the time it sits under 1-3%. I believe this server could easily get 300-400 users on it concurrently. Very nice. Another item off my list. :)
Here is the link to the server.