Hi, i need simple script that copy selected text with F6 to file on desktop mix.txt, and next time F6 add selected text at the end of that file.

You probably need a different approach for that.

Firefox extensions (such as Greasemonkey, Tampermonkey, Violentmonkey) cannot write to the desktop without displaying a download dialog, so it's not possible to write a userscript to do that, either.

The exception is those extensions which manipulate an external program (so called "native application"). Maybe there is one along the lines you're interested in. If not, perhaps an external program like AutoHotKey can do it.