If you were not trying to upvote anything, then go into the web developer tools and select the console. Repeat the process that triggered a prompt. You should see what URL is triggering an authentication request. If that does not make it obvious, close the browser and start it in safe mode. Each browser [1][2] has it's own method of starting in safe mode to avoid loading any addons or custom code.
9 comments
[ 171 ms ] story [ 1804 ms ] threadIn uBlock Origin, I had already enabled "Disable pre-fetching (to prevent any connection for blocked network requests)".
Notably, I was not redirected to the login page - which does happen if I click the upvote arrow.
[1] - https://support.mozilla.org/en-US/kb/troubleshoot-firefox-is... [Firefox]
[2] - https://superuser.com/questions/254579/how-to-open-google-ch... [Chrome]
I wasn't upvoting anything, it happened on visiting the frontpage without clicking anything there IIRC.
I'm unable to reproduce it currently, but using the console is a good idea! :)