Could you provide some more details about what this security warning
looks like? A screenshot or the text of the warning would be helpful.
OK. The proper solution is obviously to fix the app under test to not
use HTTP from an HTTPS page.
Failing that, here's how you can tweak Firefox preferences:
1. Find the preference you want to change in about:config
2. Create a new Firefox profile and set the preference:
tweaked_profile = Selenium::WebDriver::Firefox::Profile.new
tweaked_profile["some.preference"] = false
3. Create a new driver, passing in the profile
driver = Selenium::WebDriver.for :firefox, :profile => fixed_profile
4. Wrap the driver in a Watir::Browser instance
browser = Watir::Browser.new(driver)
It looks like WebDriver sets some relevant preferences to the correct
value though, see: