Hi, sites usually set a cookie showing that you completed 2FA on this device (or more specifically, this browser on this device). If the cookie is cleared by you, or by Firefox when it closes, or by an add-on, or by external cleaning tools, then the site will want you to do it again.

Can you recall making any changes to your cookie settings recently? Also, when you visit sites in private windows -- either ones you open during a regular session or in automatic private browsing ("Never Remember History" setting) -- those cookies only last until the last private window is closed.