I'm on F'fox with no problems today. When it does run slowly, I find that clearing the "Web Content Cache" sometimes helps - click "3 Bars" top-right, click Options > Advanced (bottom, left-hand list) & click "Clear Now" in "Cached Web Content" (you might have to click several times) - now restart F'fox.
If that doesn't help, you can try a browser reset. Click "3 Bars", "Add-ons", "Extensions" & make a note of those you want to use again.
In the Menu Bar (if you haven't got it showing, right-click the 3 Bars & click Menu Bar to show it), click Help > Troubleshooting Information and click on "Refresh Firefox" (top right). Then you'll need to reinstall the extensions you want.