If you've already unsuccessfully tried to resume the download then the previously partial download may no longer be present on you computer.
What you can do in such a case is:

• Move the old .part partial download and the 0 byte final file without the .part to another location.
  
• Start a new download and pause it, don't close Firefox.
  
• Copy the two files (.part and 0 byte final) that you moved above back to the download location to replace the new files that were created.
  
• Resume the download in Firefox.

1.Sometime in the halfway connection get lost and download manager shows download completed. But this file is unusable/corrupted a. Goto Tools-Download, b. Right click on it goto Download Page and re-download the same file. c. Pause Current download and open Containing Folder .You will get one complete and one .part file e.g,(i) Setup.exe and (ii) Setup.exe.part d. Now Delete .part file e.i, Setup.exe.part and rename Setup.exe file to Setup.exe.part e. Goto firefox download manager and resume download

2. During Powercut or exiting browser, Download get paused and save the downloaded files to .part file e.g, Setup.exe.part. to resume download from previous .part file a. Open download manager and goto Download Page and re-download the same file. c. Pause Current download and open Containing Folder .You will get two .part files e.g,(i) Setup.exe.part and (ii) Setup(1).exe.part d. Now Delete Setup(1).exe.part file and rename Setup.exe.part file to Setup(1).exe.part e. Goto firefox download manager and resume download

Thanx . It worked