Hello,

you can still use legacy addon version using this link: https://addons.mozilla.org/en-US/firefox/addon/always-on-top/versions/2.1.25.1-signed.1-signed
but you will have to use older firefox to install it

anyway, that's not some "separate app from a random developer":
1) I am the only developer of that app
2) You can check it with whatever antivirus you like
3) It's so small that you can't possibly put anything harmful inside
4) Please believe me - it uses pretty much the same code as in legacy addon, it just had to be moved outside

Regards,
Vasyl Maherka