Thats basically my question. I have searched the internet and I have no idea! I read in a tutorial (http://www.layeredthoughts.com/automation/how-to-write-your-first-ruby-web-bot-in-watir-scraping-weather-com) that I need to install JSSH for the tutorial to work, but there are no instructions on how to do this and I cannot find anything on the watir homepage.
You do not need to install the JSSH plugin to drive Firefox.
If I recall correctly, the JSSH plugin was needed several years ago - probably back in 2010 when the article you referenced was written. Driving Firefox, as well as Watir, have come a long way since then.
As a result, you should try following a more recent tutorial. Then go back to the article you were reading if there are additional tips you need, but would like need to adjust.
Željko Filipin's Watirbook has been frequently updated with steps for installing Selenium-Webdriver on various operating systems for various browsers. Note that Watir is simply an API wrapper around Selenium.
The installation steps are simply (for more explicit details/steps see the book):
That is it. The other plugins (Test-Wise Recorder, Firebug, FireXPath) are not needed. The latter two can be replaced by Firefox's built-in developer tools. The usefulness of recorder is often debated.
Once everything is installed you can start off with a script like:
require 'watir'
browser = Watir::Browser.new :firefox
browser.goto 'http://www.weather.com/
Then you can start working on automating the page.