It seems there are two users called passerby51, one who has been a memeber for 53 days, the other has been a memeber for 2.5 years, and has 6 questions and an answer. If both of these are you, request that the two accounts be merged.
see How do I combine my two accounts?
The procedure appears to be:
1) Have third-party cookies turned off.
2) Log into MathOverflow with the desired account.
3) Clear all cookie data for all sites ending in stackexchange.com
4) Log into ***.stackexchange with the desired account.
The order very much matters.
I haven't managed to reproduce the effect you're describing.
I'm not computer-savvy either, but this is what I see when I go to MathOverflow in a window where the history doesn't show I'm already logged in (e.g., a private window). In the menu up at the top there's a Log In button, next to Sign Up. Here I'm using FireFox 60.0.2.
(If I'm already logged in ...
Right now (December 2018), the answer is: there is a striped "speech balloon" icon on the top right of the page, and clicking it will reveal a menu with a "log out" link.
I imagine the answer to this question may continue to change over time, as Stack Exchange continues to redesign the user interface. The current answer can be found at this page: How to ...