Compatibility is mentioned in the README
Browser Compatibility
Currently, we officially support Google Chrome browser. Official support for Firefox and Safari browsers are coming soon.
There is already mentioned in different references about the supported browser:
https://support.ringcentral.com/s/article/5126?language=en_US
https://support.ringcentral.com/s/article/ringcentral-online-account-supported-browsers?language=en_US
https://community.ringcentral.com/ringcentral/topics/ringcentral-supported-web-browsers
Plus it is always advisable to use advance browsers like chrome or firefox as you will often find trouble using Microsoft browsers like explorer or edge because of it's lack of support of different plugins and libraries