Oh, that's bad. Each different attachment type should come from the server with its own Content-Type header instructing Firefox what kind of file it is. If the server is telling Firefox it's HTML or plain text -- you can use the Page Info dialog* to check on that -- Firefox will just barf it up in a tab in raw form, which is of course completely useless. However, Firefox does not have a feature where it inspects downloads and makes an independent decision about what type of content it is; the server needs to provide the correct information.

I don't know why Firefox would suddenly change behavior, since this is how it has always worked. Is it possible there was a change on the mail server side, or with an intermediary between Firefox and the server, such as security software that scans your downloads??

Anyway, I can suggest a temporary workaround, which is an add-on I wrote to fix bad Content-Type headers. If you want to give it a try:

https://addons.mozilla.org/firefox/addon/content-type-fixer/

When you first it install it, the extension is snoozing (Zzzz button). Click the button to turn it on. From that point, all .xlsx files should be associated with the proper Content-Type, which is "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet". Any improvement?

While it's listening, the extension intercepts all page requests and downloads, so it could slightly slow down browsing. I suggest snoozing it until you find a site where you need it.

* To call up Page Info, either:

• right-click a blank area of the page and choose View Page Info
• (menu bar) Tools menu > Page Info

I thank you for your reply. Problem is when I open the same page on Edge (witch I really do not want to) the attachment work just fine. It is in a program I work with from the office I work for. So its Firefox related to my point of view.