hello, all your personal data will stay and be migrated to the new version. if in doubt you can always create a backup of your profile folder though: Back up and restore information in Firefox profiles

Note that with such a major update from Firefox 3.6.x to Firefox 19.0.x it is best to start with a new profile and be very restrictive with preserving data from the Firefox 3.6.x version.

All extension would have to be updated and a lot of pref settings have been changed and are no longer supported.
For optimal stability you should just start with a fresh profile folder.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Actually no, updates from 3.6 to 19 don't require new profiles, just go ahead and update, your data will be preserved :)