Create a successor to the Telify plugin using WebExtensions

Description of the project: the Telify plugin for Firefox has not been updated for Firefox Quantum (WebExtensions API) and it doesn't have a free license (it is CC-BY-ND so it can't be included in a free operating system like Debian's main archive). The upstream web site has also gone away, it can be found here in the wayback machine / There are similar projects like Skype-for-Chrome extension and TBDialOut for Thunderbird. Your goal is to explore the source code of all these extensions and develop a new extension using the ?WebExtensions API so that it will work in the latest versions of Firefox Quantum and Google Chrome.