Configuring Firefox

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Hello. I'm trying to configure firefox in order to download files into
$HOME/Descargas and not Desktop. So, I've copied .mozilla from my model
user to /etc/kde-profiles/p1/.mozilla and this folder is copied to the
user's home directory on every session. But, I don't know how to modify
prefs.js in order to take the value I want.

I've tried with:

user_pref("browser.download.defaultFolder", "$HOME/Descargas");
user_pref("browser.download.dir", "$HOME/Descargas");

But it doesn't work.

I've tried with:

user_pref("browser.download.defaultFolder", $HOME/Descargas);
user_pref("browser.download.dir", $HOME/Descargas);

But it doesn't work.

Coul you help me?
Thanks a lot,
Carlos
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