检查这一点，可以帮助您

https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems

重置Firefox是最好的事情，其他明智的检查附加功能安装在您的浏览器

English Translate;

Download sometimes fails, then click retry attempts to re-download the Download Manager will be a big number, download time and file size are outside the normal range, similar

Reply;

Remove the download manager, close Firefox, then reinstall it. If there is still a problem, you may need a different downloader.

答复 ；

下载管理器中删除，关闭火狐浏览器，然后重新安装它。如果那里 仍然是一个问题，你可能需要不同的下载程序。