browser.download.lastDir.savePerSite controls whether the directory preselected in the file picker for saving a file download is being remembered on a per-website (host) base. If set to true, the data is stored as content preference.

Values

true (default)
The last used directory for the website (host) serving the file for download will be preselected in the file picker. If no download directory for the current website has been stored, browser.download.lastDir will be used.
false
The last used directory for any download (stored in browser.download.lastDir) will be the preselected directory in the file picker.