Hey there, which Firefox version are you on? Try to update your Firefox to the latest version and see if the problem remains.

Hmm the web whatsapp site is reportedly looking for Firefox 115 or later and since you have the eol Windows 7 it appears you are using the current Fx 115.30.0esr for it.

Maybe a proxy or vpn connection or a security software inserting its own certificate? as your post is a bit limited on information.