How to add a Cross-browser web extension

[giSe98]

I'm trying to add a simple extension but it is not seen by the browser (I added the extension both as crt and as a folder).
Would anyone know how to show me the steps to take to add the extension and possibly also let me have an example of a working extension?
hello-world.zip

Looking at my system extensions are found in ~/.local/share/midori/extensions/ and at least the default ones are all .xpi as in Firefox and its forks.

The .xpi file extension format is actually a .zip. The hello-world link actually points to a RAR archive, despite its .zip file extension. To install it you'll have to extract the contents and then repack it as a true zip archive, then change the .zip extension to .xpi.

[giSe98]

Ok, but just put the .xpi file in the extensions folder ?

Yes. After that you open the browser, go to Menu > Preferences > Extensions and enable it in the list.
(Menu being the "hamburger" button at the top-right of the main window)

[giSe98]

It doesn't work. Do you have an email so I can illustrate the problem better?

Try [...]
(removed e-mail address to prevent spam)