selenium rspec features not running on linux

I'm a rails dev working on a rails 4.0.4 app. As of yesterday (possibly before, yesterday was the first I noticed it, because I usually use CI), my archlinux dev machine doesn't run rspec features marked with js: true metadata tags, it just returns passes for all of them. e.g.:

$ be rspec spec/features/activity_spec.rb
..........

Finished in 0.38366 seconds
10 examples, 0 failures

There are no such problems for any other specs so far as I can tell, just those that use selenium. It does not spawn a browser (we use firefox, though I tried chrome with chromedriver). It seems to not even call the Procs created in spec_helper, as raising an exception doesn't happen:

Capybara.server do |app, port|
  raise "Hell"
  require 'rack/handler/thin'
  Rack::Handler::Thin.run(app, :Port => port)
end 

# use BROWSER=safari,chrome,etc
browser = (ENV["BROWSER"] || "firefox").to_sym
Capybara.register_driver :selenium do |app|
  raise "Hell"
  Capybara::Selenium::Driver.new(app, browser: browser)
end 

Updating to the most recent capybara/selenium-webdriver/rspec does not change anything, nor does checking out old tags from my repo, which used previous versions of gems/ruby.

The rest of my team (all running OSX) have no problems with the exact same branch/set of gems/ruby version (these same specs ran previously on 2.1.1 and 1.9.3 on the same machine).

All of this screams "OS specific problem" at me. Any suggestions on what to try (other than switching to OSX - have had enough of that from my colleagues) would be appreciated. Cheers.