I would like to know if my laptop is bitlocker enabled, and I use it to access internet based services through public wireless connections, can a hacker steal cookies from me and pose as me despite the bitlocker encryption?
Yes. Bitlocker only helps when the system is off. You can't start and read the disk unless you have the password. In your case, the system is already on so things can be read from it. Cookies could even be stolen over the air during your session so no amount of encryption on the hard drive helps you there.