Do you have any extensions installed that run on every site (for example, an adblocker like uBlock Origin), or are other apps (especially games or streaming) running on your phone as well? Those are the most common reasons Firefox has been slow for me. If those aren't the problem, then try clearing the app's cache: Settings -> Apps -> Firefox -> Storage & cache -> Clear cache. If that still doesn't work, you may have to uninstall Firefox and either reinstall it or choose a different browser.

Hope this helps! - TheComputerCrasher