Posting for my friend again.  I can find what model and what OS he has
if it matters.

He noticed that his version of Firefox was pretty old, so he dl'd and
tried to install the latest version.  A message said that he had to
uninstall his current version first, so he did.

Then when he tried to install the new version, it said that he didn't
have enough "privileges"  What to do??

It was a second-hand computer and he has the password of the previous
owner, but either has never needed it or hasn't needed it for years (I
don't know which)

I only know Windows.  Are not enough  priveleges something like not
being logged in as the Adminstrator?   He isn't given a choice of
"personas" when he logs in .

Thanks.