Hello,
Since Firefox 3.5 this preference is superseded with setting browser.sessionstore.max_tabs_undo and browser.sessionstore.max_windows_undo to 0.
See http://kb.mozillazine.org/Browser.sessionstore.enabled
Should add this to in the page https://portableapps.com/support/firefox_portable#performance
My question is : is it necessary to do all that is described in this page https://portableapps.com/support/firefox_portable#performance if I set the value RunLocally=true in the FirefoxPortable/FirefoxPortable.ini file?