By default, Firefox saves files to the ''Downloads'' folder on your system for unspecified files, i.e. files that do not have a (proper) MIME type specified by the server - unless you change that setting. Firefox saves an unknown file rather than allowing Firefox to process "some" file in "some" way, which would make that a security hazard. You can change the Downloads folder to another location under Options > Files and Applications > Downloads (or about:preferences#general), or set that to ask every time. For files that ''do'' have a MIME type specified, the actions should be set properly for ones that are known or added to Firefox, which you can view and change under Applications on the same page below Downloads. My understanding is that you would like to have the invoice listed there and have the ability to preview, similar as for PDF files, correct? If you see an entry for the file type involved (the invoice) and an action that Firefox does not respect, or for deleting an entry or other issues, you can reset that list by removing the ''handers.json'' file in your profile as explained in the [[What to do if you can't download or save files]] article. Also see the [[Firefox options, preferences and settings]] and [[Where to find and manage downloaded files in Firefox]] support articles. Please report back if this helped or if you need further assistance.