[dkonofalski]

That's only because you personally don't download it. I can't tell you how many times my uncle has installed a new "search toolbar" for Firefox that I've then had to disable. Luckily, disabling an extension is trivial. For the non-savvy user, though, it's still an issue.

[soapdog]

thats why the process changed and now extensions need to be signed and reviewed.

Firefox stable will not install add-ons from outside the Mozilla add-ons site by default

[dkonofalski]

Unless you change a single setting that allows you to do so. Guess how many of those malware toolbars pop-up a message that says "Click here and select this option to allow us to fix the issues with your system"?

[heinrich5991]

You simply can't disable it in future versions of Firefox.