The about:config page has that literal address -- no http/https. If you type or paste about:config in the address bar and press Enter to load the page, you should be able to find it. Does Firefox show an error message?

More info: Configuration Editor for Firefox

no it doesn't show an error messsage, just a bunch of infos pages on it, but no way to load it. I was just try to get the following : security.tls.version.max

It sounds like you are getting search results instead of the internal page. This is what you are looking for (on Windows):

I suggest pasting the exact address about:config into the address bar, making sure that it is not modified by any suggestion, and pressing Enter to load that.

yes looks like that is the one I need, and with a score of 4 on: security.tls.version.max to be able to access a certain app.

OK thanks, I got it, it is 4 as I needed.