First of all - congrats on the offer!
Most companies will pressurise you to provide a response asap, but you have the upper hand in this negotiation. They have chosen you, and now it is your turn to choose them. Although the stanstard for providing a response in the industry is 1-2 weeks, from my experience, it is doable to extend this by 1-1.5 weeks.
Having said that, you should be tactical about how you handle this:
1) Tell the other firms that you are interviewing with that you have an offer from a competing firm, in order to bring forwards your interviews and provide a response on existing rounds asap
2) Find out the deadline for providing a response to the firm that has given you an offer already
3) Ask them to extend the deadline, in order to think through the offer / finalise the interviewing process with the other firms... Although you can be honest, try to keep them warm. You can also delay providing a response, by setting up a meeting with HR/ a team member to go through any questions that you may have on the offer