The page doesn’t give any information as to why it was removed.
https://addons.mozilla.org/en-GB/firefox/addon/keeweb-connect/ and it didn’t disappeared from Chrome https://chromewebstore.google.com/detail/keeweb-connect/pikpfmjfkekaeinceagbebpfkmkdlcjk?pli=1
Is there a way to install an old version of the add-on or to know why it disappeared?