I-Firefox Isithangami Sokwesekwa

Hi there, I'm hoping someone can help with this. I recently signed into a shared computer with my personal Firefox account, and it automatically imported all my history, my browser extensions, and bookmarks, including personal/secure information that I wouldn't want on a shared computer. I tried signing out and deleting the sites and login information, even going so far as to completely delete all of my browsing history and cookies and signing in with another account, but all the supposedly deleted history and logins and extensions were then transferred to this second account. I have deleted all my data, even uninstalled and reinstalled Firefox, and all the data from my personal account keeps haunting me as suggestions when I type in the task bar or suggested usernames when I log in to websites. How can I actually delete this information so it's not on a shared computer? Thank you!