I also use Firefox and had the same problem. My WhatsApp web was being displayed in Spanish, despite my browser language was set to English. I finally fixed it by logging out of WhatsApp web and linking the device again from my phone, but first making it sure that my phone's language is set to English.

Understanding the Context

It turned out that the problem was that I used to change my phone's language from time to time. The phone's language setting is followed by the browser web version, according to this help ... Yes, per this WhatsApp page: If you changed your phone number and kept your same BlackBerry, you can change your number within WhatsApp. Before proceeding, make sure that your current number is activated on your phone and that you can receive SMS messages and phone calls.

Key Insights

Using the Change Number feature in WhatsApp will: Migrate your account information (including your profile information), groups and settings from your old phone number to your new phone number; and Delete the account ... Can I regain my old WhatsApp account after I changed my phone number ... I constantly get "Phone Not Connected" in Whatsapp Web. This issue is sporadic and resolves itself in the course of a few minutes. When I really want to send a message right now, I fix it myself by...

Final Thoughts

I believe Whatsapp Web should have an option but there's neither an option to start a new conversation with non-existent contacts, nor to add a new contact to the smartphones contacts, nor to the inner whatsapp contacts list. Has anyone a solution to contact this people over Whatsapp from my same whatsapp number?