We are frequently using the following code pattern in our javascript code

if(typeof(some_variable) != 'undefined' && some_variable != null)
{
    // do something with some_variable
}

and I'm wondering whether there is a less verbose way of checking that has the same effect. According to some forums and literature saying simply

if(some_variable)
{
    // do something with some_variable
}

should have the same effect. Unfortunately, Firebug evaluates such a statement as error on runtime when some_variable is undefined, whereas the first one is just fine for him. Is this only an (unwanted) behavior of Firebug or is there really some difference between those two ways ?