I would suggest you ask someone that knows Spanish. I don't. Perhaps try asking here https://support.mozilla.org/es/questions/new/desktop/form

Te sugiero que le preguntes a alguien que sepa español. Yo no. Tal vez intente preguntar aquí https://support.mozilla.org/es/questions/new/desktop/form

That message is from Firefox's Portal cautivo feature.

The main purpose of this feature is to help you sign in to free WiFi networks that require accepting their terms and conditions before you can browse. But often your device will handle that automatically and you don't need Firefox to do it.

If you would like to disable this feature:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Editor de Configuración para Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future. Since we have had this setting for a long time and I use it myself, I feel comfortable mentioning it.

(2) In the search box in the page, type or paste network.captive-portal-service.enabled and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

If you have problems browsing after this change, you can double-click again to switch the value back to true.