The "do this automatically for files like this from now on" checkbox does NOT have any effect for me. Every time I open an attachment of the same type (e.g. .zip files) it still presents the dialog box asking me if I want to "open with" the related viewer program (e.g. Archive Utility, by default). I'm using an iMac with OS High Sierra, and I have the most up-to-date version of Firefox.