Mozilla does not offer a dedicated telephone number.

Their primary method of support is through their online Mozilla Support Forum, where a community of knowledgeable users and some staff can help with issues.

https://www.mozilla.org/en-US/contact/

The only direct support Mozilla has is by ticket support for their subscription based products as in Mozilla VPN, and plus/premium versions of Relay, Pocket, Monitor, MDN Plus as part of the subscription.

How would Mozilla pay for the call centres that would be needed to do support for free to download and use Firefox web browsers, even if they only do North America and not globally.

All web browsers do user support by some form of forum as direct support may only be for say enterprise users who pay for it.