Is that the only place that happens? Since that is a login site no one else is going to be able to view and replicate what your describing here since they don't have access to that location.

That is a standard tooltip that sows the content of a specific field as set via the title attribute. If the tooltip wasn't there before then the website might have modified their page and add this as the title attribute. You can check that via the right-click context menu in the Inspector.

You can right-click and select "Inspect" to open the built-in Inspector with this element selected.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/open_the_inspector/