Slowness could come from several different sources. One quick improvement is to use AdBlocker, or even block Javascript if your workflow allows it. This makes browsing extremely fast. Another one is missing support for upcoming and non-standard web technologies, that usually are implemented in Chrome (Blink) first, that leaves Firefox (and other browsers) using slower performing polyfills. As much as I hate the direction Firefox UI is going, there are no decent browser alternatives, especially those who support adblockers at such low lever as Firefox.