It was only added to Firefox addons site recently but one could always install it manually for a long time. On Chrome, it has 2000+ users - https://chrome.google.com/webstore/detail/falcon/mmifbbohghe.... It works for me since I only use it to search recent history
As for Firefox, you can right-click the "Add to Firefox" button and save and inspect the extension.
It's not that I don't trust it but that the value of this extensions looks to be on a long period, you need it to be maintained for months/years to be useful.