Based on what Yahoo is saying on its site (see attached screen shot), it sounds as though it is saved in a cookie, which means that it is specific to each browser, since browsers do not share cookies between them. And of course it is subject to getting zapped for the same reasons that other cookies might get cleared.

So... how do you feel about keeping cookies between sessions?

Specifically, it appears the information is stored in the YL cookie from the login.yahoo.com domain. Either of the following add-ons can delete all but certain cookies, for those who regularly clear out cookies but need to keep a few.

• https://addons.mozilla.org/firefox/addon/biscuit-220876/
• https://addons.mozilla.org/firefox/addon/selectivecookiedelete/

See also:

Fix Sign-in Seal problems: https://help.yahoo.com/kb/yahoo-account/SLN2676.html