Windows Registry Editor Version 5.00
@="\"C:\\Program Files (x86)\\Mozilla Firefox\\firefox.exe\" https://keep.google.com/"
The above registry edit should work for keep+firefox. It does for me, at least, although I registered it on HKEY_USERS too, just to be on the safe side (but I have no idea how to know the user registry ID without being in front of it)
you can also pretend you have an app by installing google chrome, the google keep extension, and open it as a "standalone" app. The url looks something like
"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --profile-directory=Default --app-id=hmjkmjkepdijhoojdojkdfohbdgmmhki