Is a NPAPI plugin what I am looking for?

I need to develop a mini ClickOnce plugin for Firefox, Chrome, Operah and Safari (preferably, one that works on all of the above).

My plugin needs to be able to run an application in a one-click fashion. In other words, by going to a specific URL, my application will run on the target machine (no prompts / dialogs shown). Unfortunately, ClickOnce is only available with IE, and I am unhappy with the ClickOnce plugins that currently exist. I would like to develop a tailored ClickOnce plugin that suits my specific needs.

I read about FireBreath, which sounded like a perfect solution for me. I also read about the difference between an extension and a plugin and I think a plugin is what I am looking for. I just wanted to make sure with you guys that I am on the right path. Is what I want to achieve possible with FireBreath / NPAPI plugins? Can a plugin download an executable and then run it on a target machine?

Yes, NPAPI will allow you to do what you want. You can run arbitrary native code using an NPAPI plugin, and control it via JavaScript on the web page side. Users will have to download and install your plugin though, so it only makes sense if this is something users will use more than once.

You'll obviously have to be very, very careful about security though. If you make a plugin whose sole purpose is to download and run code without user interaction then you'll need to be absolutely certain that there's no way for a malicious page to use your plugin.