You don't know if a file you just have downloaded is corrupted or not. If the originator of the file provides a checksum (hash total), this extension can calculate the checksum of the downloaded file and by comparing it with the original checksum (for now, you must manually copy and paste it), check if the file has been corrupted.
This extension can calculate MD2, MD5, SHA1, SHA256, SHA384 or SHA512 checksums of downloaded files.
Md5 Reborned Hasher isn't compatible with the new download tab in the Firefox library, if you wanna know more about this problem see bug 8 of md5rehasher in sourceforge project.
For now, to get it to work in the latest version of Firefox you need to change the value of "browser.download.useToolkitUI" in about:config to true.