Unfortunately, there is no way to ask for the filename first. Firefox only allows to show the save dialog when the content is available as a single file/URL. The only workaround I know is to turn off the dialog in the options.

扩展需要下载到内部存储，全部完成后才会复制到下载文件夹。火狐57以后的扩展没有直接读写文件的权限。

请更新扩展。
旧版本已发现火狐75不兼容，详见 https://github.com/jingyu9575/multithreaded-download-manager/issues/128 。
请注意，现在退出浏览器前必须手动暂停所有下载项目。

It was found that the extension is incompatible with Firefox 75. See https://github.com/jingyu9575/multithreaded-download-manager/issues/128 for details.
For now, you can change the storage API to Segmented files in the Advanced tab in the options. This option only works for new downloads, so if you have a very large existing download, I suggest downgrade to 74 to complete it. Also, with this API you need to manually pause all downloads before existing Firefox, otherwise some progress will be lost.

已发现火狐75不兼容，详见 https://github.com/jingyu9575/multithreaded-download-manager/issues/128 。
目前请手动将 选项-高级-存储API 设为分段文件。这个设置仅应用于新的下载，所以如果目前正在下载较大的文件，建议降级到火狐74来完成。另外，此模式下退出浏览器前必须手动暂停所有项目，否则会丢失部分进度。

Firefox Nightly 74 just removed an important function (getFile) that the extension depends on.
The extension has been updated to workaround this issue, but the performance of saving files will decrease.
Additionally, it is unknown whether Firefox will remove the whole mutableFile API in the near future. If so, the entire storage layer will need to be rewritten. It may or may not be possible to build this extension with acceptable performance then.
I'll see how things go and prepare workarounds if possible.

Firefox 57+ extension cannot do this currently. You can probably move this directory to another disk and then create a "symbolic link" back.