worked to enable Google Voice calling with any numbers, just had to set the URI scheme to "tel" and set voice.google.com as the default for the "tel" protocol in Firefox settings

It will work, if you choose the right settings: for Bria 5 I use 'tel' or 'sip' as the protocol and Europe for the region, when mostly using German websites. This works well for all kinds of numbers, but it interferes with some websites.

Some phone numbers are not recognized and a right-click menu option to select such a number would be helpful. Also excluding some sites using a blacklist would solve the problem of interference.

Давно искал для обновленного Firefox. Наконец-то всё работает!