Along with the suggestion to use "that-which-shall-not-be-named"...
I find that clearing the "Web Content Cache" sometimes helps - click "3 Bars" top-right, click Options > Advanced (bottom, left side) > Network (3rd tab, top) & click "Clear Now" in "Cached Web Content". Let it do the bit of working-out it needs, now restart Firefox.
If that doesn't help, you can try a browser reset. Click "3 Bars", "Add-ons", "Extensions" & *important! - make a note of those you want to use again, along with any settings you applied to them & might forget*.
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.