Try to use the right-click context menu and use "Save Link as" if you are currently left-clicking the link. Firefox doesn't know that you want to down;load the file and first opens a new tab.

You can also consider to set this pref to true on the about:config page and hold down the Alt key when left-clicking.

• browser.altClickSave = true

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

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

• https://developer.mozilla.org/Mozilla/Preferences/Preference_reference/browser.altClickSave

doesnt solve the problem

Firefox shouldn't open a new tab when you use "Save Link As".

Did you try to use this item in the right-click context menu?

How does the server send these files?

• https://developer.mozilla.org/Tools/Network_Monitor
• https://addons.mozilla.org/firefox/addon/live-http-headers/

Are they opened in the new tab if you left-click or do you get an empty page with the open with download dialog?

Another user reported a "blank tab left behind" problem when downloading MP3 files in this thread: Firefox keeps opening new tabs when I download files. Refreshing Firefox will temporarily halt that, but the problem returns the next day I boot up. A workaround was to disable the multiprocess feature, as described in that thread. Could you test that?