It seems Selenium Webdriver gets frequent updates to keep up with Firefox. But how do you know which version you need?  Hopefully this procedure will work even as versions change:

1. Go to http://www.seleniumhq.org/download/.

2. Scroll down to Selenium Client & WebDriver Language Bindings.

3. In that section, in the Ruby language line, click on "Change Log" (direct link).

4. In the Change Log, determine which version of Selenium you need for your version of Firefox.

If you're using Bundler, run bundle show selenium-webdriver to see which version you have.  To update, for example to 2.35.0, add this line to your Gemfile:

gem 'selenium-webdriver', '2.35.0'

and then run bundle update to install.  If you are using Spork, remember to re-start it before re-running your tests.

Update One StackOverflow answer indicates that the Change Log may be updated sooner in the source code repository than at seleniumhq.org.  The repository Change Log for Ruby is here:  https://github.com/SeleniumHQ/selenium/blob/master/rb/CHANGES.

Downgrading Firefox

If you need to downgrade Firefox on Ubuntu 12.04, this answer explains how to go back to Firefox 20.  A more general description of a way to switch to any version of Firefox is given here.  Then use this answer to put Firefox updates on hold until Selenium releases an update that works with the later version of Firefox.

In my case, I downgraded Firefox only to discover that Selenium Webdriver had recently been updated to handle the latest version, so check for Selenium updates first!