This works. However, don't use it if you install a similar one for Chrome/Chromium, and you enable synchronization of GNOME extensions in Chrome/Chromium (as per https://www.omgubuntu.co.uk/2017/04/sync-gnome-shell-extensions-google-chrome). When I used both, then GNOME integration for Firefox uninstalled GNOME extensions that I had installed in Chrome.
That is expected. Firefox extension is using Firefox sync, Chrome extension is using your Google account. There is no connection between 2 sync accounts. What did you expected?
