Fully delete bookmarks and passwords in a Galaxy tablet

I've got a new Galaxy tablet S10, and I planned to use Firefox. Starting off, I opened a website, bookmarked it, entered and saved password data, and followed through with 2-factor authentication. I decided that I didn't like what I had done and started over. I deleted the password data, deleted the bookmark and started from the beginning. PROBLEM: When I opened the website again, Firefox automatically opened it to my own account COMPLETELY -- not just the website opening page, but my own account page, INCLUDING EVEN the results of the 2-factor authentication. What am I doing wrong?