Failed downloads can get deleted if they fail to download. If you have such problem you need something like Free Download Manager to help with downloads that have internet issue to restart but if the download links are static there's noting much download manager can't help either.

For large files I suggest using software specifically designed for downloading, particularly because of your internet connection problem. I don't know what might be available on Windows 10 but you should be able to find something. One I know of (available online) is JDownloader.

Thanks everyone for answers.

While the problem still remains and I believe this should be a configurable feature, but I have used a workaround. Here it is if anyone finds it useful:

1. Pause your active download (mouse right click on download opens a menu) 2. Copy the .part file somewhere else in your computer 3. Resume your paused download (mouse right click on download opens a menu) 4. When your download gets interrupted and Firefox deletes .part file, simply restart the download from zero. 5. Pause your download again. 6. Rename the old copied .part file to be the same name as the new .part file 7. Replace the new .part file with old copied .part file 8. Resume your download and Firefox will continue download at the progress of your copied .part file.

That's alot of work just to download. The downloader software does that so user doesn't have to do this if everything was copied without errors.

That is similar as what we suggested in the past when a download failed because of an unstable internet connection.

The usual procedure where you want to resume an interrupted download is as follows: Make sure to backup the .part file and the final file with size 0 without the .part before starting Firefox. Try these steps to resume an unfinished download if you still have the .part file.

1. Move the old .part partial download and the final file without the .part that has size 0 to another location.
2. Start a new download and pause it, don't close Firefox.
3. 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.
4. Resume the download in Firefox.

If that doesn't work then the server may not support resuming.

https://support.mozilla.org/en-US/search/?w=2&product=firefox&q=%22Make+sure+to+backup+the+.part+file%22