You can pause the download if you know when you'll lose connection but auto-pausing is hard if your connection keeps dropping.

but it's possible right? auto-pausing, I can't watch my internet, I leave on download and go to school or something like that, When I came home it's just downloaded 10 mb D:

If auto pause not possible then how can I download files from browser using internet or there any download manager that have auto pause option?

If your connection got interrupted or Firefox crashed than it is possible that the built-in Firefox Download Manager won't resume the download and start at the begin.
In such cases it is better to use an external Download Manager to resume and finish the download.
You can rename the unfinished file to the correct name by removing the .part file extension and point the external download manager to that file.
That will only work if the server supports resuming.

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.