If you use this feature frequently and can't find another add-on that makes it easy, you could consider switching to the "ESR" version of Firefox, which is designed for organizations that require long-term feature stability. The ESR version is currently based on Firefox 24 and gets security updates when the regular version of Firefox gets full updates. More information: http://www.mozilla.org/firefox/organizations/.

Downloading and burning Linux or Windows ISO images for example with a single flipped bit can cause system instability nearly impossible to track down, and not otherwise detectable. An MD5 checksum (OR SHA) check will quickly detect any such transmission errors. Not something anyone I know bothers to use unless it's important data. The temporary fix of changing the about:config setting, then expanding to full screen in downloads was working. But now, based on my interpretation of comments made on the authors Sourceforge page, Firefox has made updates that prevent the MD5 Reborned Hasher from working in Firefox at all. If that's true, I would really like to know the reason. With all this talk of NSA spying, I would think more people would be interested in making sure someone isn't tampering with their downloads. I have no idea. But I do believe the wonderful volunteers at Firefox need to figure out a way to make this valuable piece of security software whole again. Thank You.

PS: For those needing to verify a file, you might want to try the "Microsoft File Checksum Integrity Verifier" kb841290 . It's old, but does large file MD5 from a command prompt.

Hi Suncoast, the initial problem was that all extensions which modified the UI of the old Download Manager window became obsolete starting in Firefox 26 (or if you stuck with the default settings, in Firefox 20). There might be a way to integrate it into the new downloads list, or the recently released extension which creates a different download window, but I don't know whether anyone is working on it.