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.