Error Message Says FileZilla Will Damage My Computer
I recommend binning the app in this case. Get a new copy or different tool.
The FileZilla downloads and updates now contain PUP - potentially unwanted program (or PUS if software makes more sense than program). The line between malware, adware and privacy stealing information is blurred and when a package has these modifications you will sometimes be prompted with a warning. Even when the owner of a tool decides to monetize their tool, Apple is generally flagging that change with a signing error so people know a common tool isn’t what it was in the past.
This warning can be as simple as the application is modified and what was signed isn’t what is running anymore.
Many people have repackaged this app, so be sure you do the work to get a clean copy or perhaps delete it and redownload. I love Mountain Duck and cyberduck for this if you find the zilla hassle too great to get it reinstalled.

https://medium.com/web-design-web-developer-magazine/how-to-safely-download-and-cleanly-install-filezilla-ftp-software-with-no-additional-junk-10b27a2d270d
https://cyberduck.io/
https://mountainduck.io/