I figured I'd comment on this because my family has been having this problem (I keep on saying "cancel" whenever it prompts me to download 3.6.16 because of it).

I did some research on the WS.Reputation.1 plugin-container.exe problem and found that it was happening last year with updates, too. To fix that problem, people took the quarantined file and restored it- I was hesitant to do this because I wasn't sure if it was a virus or not.

I took it out of quarantine on a family computer and right away firefox started working again. Again, I do NOT know if the file is a virus but by doing research I found that it is not, it is only recognized as such by Norton. This seems to be a recent problem and I don't want to take ownership for saying how I fixed the problem because, again, I do not know if WS.Reputation.1 is really a virus but all indications are that it is not. I hope this at least helps some of you. There have been no problems since I took it out of quarantine and restored it. Good luck everyone.

I scanned file WS.Reputation.1 and it did not contain a virus. I took it out of quarantine. I uninstall FF4 and reinstalled it with Norton's disabled. This time, the file did not get quarantined. FF4 worked good yesterday, but today it's back locking up. I had no problem with my previous version of Firefox and am thinking about going back to version 3.5 or 3.6 until Mozilla comes up with a solution. For now, IE 7 works great! (and, with Norton's)

Norton was reporting a false positive for the plugin-container.exe file in the Firefox 3.6.16 update.