Unfortunately not unless you can click "resume" and it is able to successfully resume the download. Otherwise you need to just start over.

Do you still have the .part file?

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 when 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. Best is to use an external download manager to prevent issues with paused or interrupted downloads.

Thank you, but unfortunately the server doesn't support resume download.