You don't need to submit and finalize until you are ready to. There is no save button; Bonfire automatically saves your progress on the fly as you work!
The next to time you log in, simply go back to the Project you were submitting to by clicking on the corresponding View Opportunity button.
Then, scroll to the Submission section near the bottom of the page. Here you will see your Work-in-Progress submission(s). Click the Continue Submission button to resume exactly where you last left off!
Alternatively, you can click on Submissions in the toolbar of your portal.
From there, you can navigate to your Work-in-Progress Submissions and select View from the Actions drop-down menu to continue the submission you previously started.
If you've already finalized and submitted and need to modify your submission, see Can I un-submit my bid once it has been submitted and finalized?