The command line pref for global installation of extensions was removed from Firefox 3.6 and later versions. Gecko 1.9.2
http://www.mozilla.org/docs/command-line-args.html#-install-global-extension_.2fpath.2fto.2fextension
Drop the extension XPI file into the Firefox program files \extensions\ folder. It will install into each Profile when that Profile is launched.

I tried this with FF4, but nothing happend? I used: Forecastfox Weather Site Information Tool What's wrong?

@the-edmeister:

Ed, thanks for your reply, but in fact it is a reply to a different question - in fact, the question which, as I said, I was originally searching ("How do I install Addons for all users?"). That leaves two things to do:

1) Since 50 users have already requested an answer to that question <http://support.mozilla.com/en-US/questions/745665>, would you please post your answer there? If you don't want to do that, may I do so? [It would be better if you do it, as you clearly know a lot more about the issue (and might be able to answer follow-up questions, of which I have at least one already).]

2) We still need an answer to either:

  a) what is wrong with the KB search here or
  b) what is wrong with my search procedure?

Thanks, John

You might be better off posting your question to the dedicated add-ons forum @ https://forums.mozilla.org/addons/ if your question is really about installing add-ons for all users rather than about how the KB works.

@Xircal: Again, like the-edmeister, you have answered the question "How do I install Addons for all users?" <http://support.mozilla.com/en-US/questions/745665> - which I am trying to get moved to its proper location - not the question asked here. And again, your answer, like Ed's, is helpful, so could we please move those answers there? - and then get to the more basic question of how to get the "Search Firefox Help" function to work properly? Thanks,

John