You can look at the solution posted by Nima in this thread: [tiki-view_forum_thread.php?comments_parentId=329282&forumId=1 how to resume firefox 3.0.8 a crashed download from previous part file ?]

okay i made this account jus to tell u this There is something you can try when this happens..

Go to your downloads folder location. If the download has failed, Firefox may have created a .part file. If your file is largefile.exe, look out for largefile.exe.part.

Now basically all you have to do is go to the downloads page and start downloading your file again. This time pause the download, go to your downloads folder.

Rename "largefile.exe" to "oldlargefile.exe". Rename "largefile.exe.part" to "largefile.exe" obviously replacing the name with your own file.

Now resume your download. It should continue from where it left off.

This is a nice trick. Can FireFox include this as a routine option?

A correction to the details of the instructions above : You have to start the fresh download in a new folder. Then pause the download and replace the file named largefile.exe.part there with the older file of the same name from the unfinished download folder. [ If you must use the same folder for downlaoding, then put the older largefile.exe.part file in some other temporary folder before beginning fresh download.]

I saw something amazing today! It might not be a great help but, i would still like to share. I searched for the exact file name that i was downloading. I found a torrent for the same, alternatively you can search for the same file with a different name and then rename the incompletely downloaded file to the name that is in the torent. Make sure that the download folder location is the same, and tada! the file will resume its download. Torrent Client will pick the older file and start downloading