There is always a period of time after you requisition hardware that you are a little nervous about performance. Very happy to report that Firefox is scaling very well and we have jumped over the magic number of 100 concurrent users in FF.
The only open issue right now in regards to Firefox is that it still is having an occasional oddity in regards to NFS. From watching patterns, it seems like if you are downloading a big (1GB+) file and writing it over NFS, the server get's sluggish. It never comes to a crawl, but has a noticeable slowdown. I was able to make this work better by downgrading from NFS4 to NFS3. I'll see if I can figure it out and submit a proper bug report.
The shots below are 100 concurrent FF sessions. Not everyone is surfing at the same time, but it still shows the memory usage along with CPU for maybe 30ish active sessions. Very happy to see Flash not multi-threading. :) I believe this server could easily handle another 100 users, and can easily be upgraded in terms of RAM.