Attachments will always download.
If you do not choose for the file to be saved, it will be downloaded to the Windows temporary memory.

You can choose to open the file and this will cause it to be saved in temporary memory. You can also select to save the file which will save the file so you can reopen it.

Windows temporary memory is deleted periodically.

Are that PDF files?

Current Firefox versions have enabled a built-in PDF Viewer that doesn't have all features that other PDF readers like the Adobe Reader have or may not function properly otherwise.

You can change the action for Portable Document Format (PDF) from Preview in Firefox to use another application like the Adobe Reader or set to Always Ask in "Firefox > Options/Preferences > Applications".

You can set the pdfjs.disabled pref to true on the about:config page to disable the build-in PDF viewer.

You can check the value of the plugin.disable_full_page_plugin_for_types pref on the about:config page and remove the application/pdf part if present or reset the pref to the default via the right-click context menu.

• http://kb.mozillazine.org/about:config

See also:

• https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them