OnChrome needs a native application to connect to Chrome. Download it here.
The only way for an extension to connect to an application on your machine is via Native Messaging. OnChrome needs to do exactly that to open Chrome.
You can view the source code and compile it yourself if you prefer. I'm providing the binaries because I assume it's going to be the easiest route for 99% of the users.
Install the extension from the Firefox add-on store.
If you use homebrew, you can run brew cask install g3rv4/install/onchrome
If you don't, download and install OnChrome.pkg.
If you use chocolatey, you can run choco install onchrome
If you don't, download and install OnChrome.msi.