It might be hard to defend that as a unique enough trademark. Not impossible if you can afford the sort of lawyers that get words like “windows” and “apple” pro

It might be hard to defend that as a unique enough trademark. Not impossible if you can afford the sort of lawyers that get words like “windows” and “apple” protected, but not easy. -zilla was a fairly common suffix for various things including, but not limited to, software when the company took on the name. I expect the majority of people hearing the name filezilla will think of Gozilla far ahead of Mozilla and assume the name comes from there rather than implying a link to the company (a lot of non-techies might not even know/care that Firefox comes from a company call Mozilla) so the chance of meaningful conclusion would take quite some arguing for. Protections that would apply to -zilla as you suggest could be used for a lot else by other corporates, so be careful what you wish for there.