I wonder if Mozilla could argue that Filezilla is infringing on their trademark and damaging their reputation because of this.

I mean, it's an open-source client for a common internet protocol that ends in "zilla". It would be easy for users to assume that Filezilla is affiliated with Mozilla.

And up until the move to Sourceforge's adware downloader system, that would have been fine for everybody - they're both good products.

But now? Now filezilla is riding on mozilla's coattails with the confusion and profiting from it, to the detriment of mozilla's reputation.