where(:firstname => [first1, first2])

Notice how you can provide an array and Sequel takes care of
constructing the filter. Also gives you nicer-looking SQL:

  ... WHERE firstname IN ('john', 'george')

If you need to qualify the field name you can use the special symbol
notation:

  where(:c__firstname => [first1, first2])

Sequel also lets you use any comparison operator, not just ==:

  where {:price > 100}

You can also specify mathemtic expressions and compare against other
fields:

  where {:price < :avg[:price] - 100}

Have a look at the cheat sheet:

  http://code.google.com/p/ruby-sequel/wiki/CheatSheet