This extension may look a bit frightening at first, but with some basic knowledge of http headers (and, luckily for me, no Chinese at all) it's really easy to do simple things.

The problem that Header Editor helped me to solve is as ancient as the hills: it's that infamous bug about how Firefox handles Content-Disposition: attachment headers (https://bugzilla.mozilla.org/show_bug.cgi?id=453455). A legacy extension needed replacing with Firefox Quantum. I tried a few specialized WebExtensions with little success (sometimes they worked, sometime they didn't), then I stumbled upon Header Editor.

Here I set up a single, simple rule to "Modify the response header", match type "All", execute type "normal", to always set the "Content-Disposition" header type to a value of "inline". That's it, two minutes to set up the whole thing, and it works beautifully!

Of course Header Editor can do much more complex things. With its support for custom JavaScripting it certainly has the potential to replace a whole bunch of smaller-scale extensions for all kinds of stuff related to http headers. In the right hands it can become a very powerful tool with endless possibilities. I guess that's about the best thing one can say about any piece of software.