Can I log into firefox on somebody elses account?

Hi, no. You would have to set up a new Profile *https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles in her Firefox for your use. Setup Sync to Sync your Profile on her Firefox from your Firefox. *https://www.mozilla.org/en-US/firefox/features/sync/ Or you would need to manually backup your bookmarks on a USB and keep updating it and or keep updating the profile. Should always do this anyways : *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles *https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox Please let us know if this solved your issue or if need further assistance.