Attaching files doesn't appear to work in Firefox

[passel]

How did you try to attach?
I just used the Manage Attachment button on the "Go Advance" reply, and it appears to have worked fine.
I've been using Firefox all along, and Firefox says it is up to date (46.01) on this machine (which is a Lenovo laptop with Windows 10 on it).

Perhaps you've made some optional selection some years back that causes the issue.
Perhaps reset the options back to the default values to see what that does.

I just picked an existing zip file on this machine (which there are not a lot since it is relatively new machine and I don't normally use it to develop anything), but there were some zip files in an old VB6 directory that was obviously copied to this machine at some point.