Can I use same email ID to login into multiple Webex Connect tenants?

No, it is not possible to have multiple accounts linked with the same email id. Only one account can be created for one email id

Which browsers can I use to access my Webex Connect account?

Webex Connect can be accessed using the latest versions of Chrome, Mozilla, and Safari.

What is the difference between Dialogflow ES and Dialogflow CX pre-built nodes?

Dialogflow CX represents an advanced iteration of ES. Dialogflow ES is best suited for simpler chatbot applications, whereas Dialogflow CX is tailored for intricate, multi-turn conversational experiences, particularly in enterprise-level settings. The choice between the two depends on the specific requirements and complexity of the project at hand.

See here for more information about migrating from Dialogflow ES to Dialogflow CX.