L1b to h1b...can employer fire employee just because he is moving on h1b

currently I am on L1B with valid I-94 till 2014. I got H1B approval now and start date is 10/1/2013.
now if my company fired me then can I stay till 10/1/2013

is it valid as it will be 15 days gap?

I heard that a person can come around 10 days before in USA before start date. so in my case if start date is 1 oct 2013 then i can come any time after 20 sept. if that is the case then …is it I just need to worried about just 5 days ?

beside this can my company fire me as i am moving on H1b…?