It didn't with iceweasel; I have no idea about iceape. When I was still
using the mozilla suite I was always successful with granting temporary
write access if an extension could not be installed right away as a
normal user. You should have a file called "install.log" in your
~/.mozilla/... directory; the installer should record all unsuccessful
file access attempts in this log. Granting temporary write access to the
problematic files/directories and repeating the installation as a normal
user always worked for me. (Note, however, that this was at least 2-3
years ago; I have no experience with any recent version of iceape.)