danke für den tipp bc auch wenns leider nicht geholfen hat.
meine user.js datei sieht jetzt so aus:
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user_pref("network.http.pipelining", false);
user_pref("network.http.proxy.pipelining", false);
user_pref("network.http.pipelining.firstrequest", false);
user_pref("content.notify.backoffcount", 5);
user_pref("plugin.expose_full_path", true);
user_pref("ui.submenuDelay", 0);
user_pref("content.interrupt.parsing", true);
user_pref("content.max.tokenizing.time", 2250000);
user_pref("content.notify.interval", 750000);
user_pref("content.notify.ontimer", true);
user_pref("content.switch.threshold", 750000);
user_pref("nglayout.initialpaint.delay", 0);
user_pref("network.http.max-connections", 48);
user_pref("network.http.max-connections-per-server", 16);
user_pref("network.http.max-persistent-connections-per-proxy", 16);
user_pref("network.http.max-persistent-connections-per-server", 8);
user_pref("browser.cache.memory.capacity", 65536);
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