Microsoft offers its own guide to speed up the Firefox browser. You can access the link below to see the complete steps. Basically, to speed up Firefox, the best way is to keep only the add-ons that you actually need.
Secondly, go to the Options menu and increase the local Cache storage. This will greatly increase the page loading time. You can do this from the Advanced > Network.
You can increase the loading speed using about:config as it goes:
- network.http.pipelining.maxrequests 8 (replace from 4)
- network.http.pipelining true (replace from false)
- browser.display.showimageplaceholders false (replace from true)
- browser.tabs.animate false (replace from true)
- network.http.max-persistent-connections-per-server 8 (replace from 6)
Restart the browser and you should see the changes immediately.
Mozilla Guide: https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster