Форум поддержки Firefox

Long ago I used to never have to hit F5 to see today's version of a page instead of the cached version, and I do NOT want to clear cache or cookies or history on exit, no, that's not the setting I'm looking for, thanks.

I've noticed that when logging into various websites, a random email address not known to me appears in autofill among my usual email addresses. This unfamiliar address is under the @shanreto.com domain and shows up across different websites. My Firefox account is synced between my PC and mobile phone. Both devices are password-protected and under my control at all times, and I have only installed uBlock Origin as an extension.

Could there be any explanation other than the possibility of someone accessing one of my devices through malicious software or similar means?