Problem downloading book from Kobo

[bluepeter]

This morning, I bought two e-book from Kobo. I then tried to download them, so that I had copies stored on my hard drive. As usual, I went to My account/My Books, and clicked on the Download links on the right. The first dropped a file called URLLink.acsm into my Downloads folder - i.e., it behaved as I've come to expect.

The second one, though, didn't. Instead, it opened a copy of the book in Firefox. I don't know if it's relevant (though I don't think it ought to be), but the book in question appears to be DRM-free - the filename begins "macmillanuk_drmfree-epub-". This has not happened with DRM-free books in the past: they've downloaded as I'd expect.

My Firefox settings are quite clear: the action associated with EPUB files is to save the file.

I was able to resolve the problem by using Chrome instead of Firefox. But:
(a) why did Firefox open the file instead of downloading it?
(b) how can I prevent this from happening again?