Can lizamoon virus affect firefox 4.o? Have there been anyreports of infections?

No. Especially more so on Linux.

For example http://community.websense.com/blogs/securitylabs/archive/2011/03/31/update-on-lizamoon-mass-injection.aspx

A Linux (or Mac OSX) user should realize that it is a fake when it shows a supposed Windows Security Scan that shouldn't appear in a non-Windows operating system. Windows users might not spot it as a fake as easily and could be duped into downloading harmful "scare-ware" fi they are not alert.