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If downloads hang at the last minute (just before reaching 100%), the cause might be antivirus software which tries to scan the file and during that process hangs the Firefox instance.
In order to fix the problem, try disabling the scanning as follows:
- In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
- Click I'll be careful, I promise!, to continue to the about:config page. A list of settings should appear
- Filter the list by typing in the search bar at the top. Type in scanWhenDone. You should now find the browser.download.manager.scanWhenDone preference.
- Double-click on that preference to set the value to false.
- Next, try to download something and see if it still hangs.
Please reply and tell us whether this helped you!