I don't often try to get previous versions from the developer. While there are strongly ethical and consumer-centric software houses, I have been long conditioned to expect predatory behavior, usually with some degree of incompetence.

So my first inclination wouldn't be to try filezilla but archive.org. After that I'd search for other options and see what was most trustworthy.

I've done well with oldversions.com in the past. Last I heard they were still reputable. http://www.oldversion.com/windows/filezilla/

Even without knowing which version the author is looking for, I'm betting I could locate a suitable copy.