I recently attempted to open up an eBook I had downloaded in CHM format.
Upon opening it with Microsoft’s CHM reader I was presented with the table of contents on the left but on the right where the content should be an error was displayed:

“Navigation to the webpage was canceled” and
“The address is not valid”

I had seen this before and simply gave up because I figured it was a corrupted file. I then thought about how it would be neat if there was a Firefox addon for reading CHM files. A simple Google search revealed that this already existed! Introducing CHM Reader.

CHM reader opened the CHM file with no problems. My first disappointment was that there was no table of contents/navigation menu. But I was quickly pleased when I discovered that was an option in View -> Sidebar. The main advantage of CHM Reader is that it allows you to increase the text font size which can come in handy.

Back to the main point, why could Firefox display the CHM file but not Microsoft’s reader? As it turns out, on newer versions of Windows, Microsoft attempts to help you out by blocking some downloaded CHM files.

CHM Properties Menu

If you right click on your CHM file, go to Properties, and then click ‘Unblock’ next to security, you will be able to read the file with Microsoft’s tool.

Thanks Microsoft.