Reply #1: Maybe it's not you. I have 3 ideas you can try. (the first 2 are serious.)

[Richard Steele]

1: Download it again, after making VERY certain that you are
on the REAL FireFox homepage, not a hacker-run mirror site
posing as the real thing. (That really does happen, you know)

Also, bear in mind that downloads can be garbled in transmission;
a single one or zero out of billions gets lost,
and suddenly the whole program is junk.
I download LOTS of stuff, and have found that "bad" programs
work fine about 50% of the time on the second try.
Download it again.

2: Disable Norton for a minute while you install FireFox.
Then turn Norton back on.
Even if Norton still cries foul, it might give you some
"ALERT" messages which would provide some insight into the problem.

3: My standard "Idea #3" requires a baseball bat and
a bank account large enough to purchase a new computer.
:argh:
Given the current state of the economy, #3 is generally regarded as a bad choice
for all but the most advanced users.