From a QA email thread:

It should not be hard to do. You do not have to set up a profile, you can just point to the extension that needs to be installed.
https://code.google.com/p/selenium/wiki/RubyBindings#Adding_an_extension

This may be the perfect opportunity to refactor the UW test suite to
make use of mediawiki_selenium 1.0.x features—it provides a dependency
injection system for easily setting custom Watir/Selenium browser
options.[0][1] It would go something like this:

Before('@use-some-extension') do
  browser_factory(:firefox).bind do |options|
    options[:profile].add_extension('/path/to/extension.xpi')
  end
end