I'm a long time Firefox user but got very angry at it last week when it updated to the newest release, Quantum (v. 57.0). It would freeze my screen, would not let me close it, was very slow with tab and page changes. Thought it was my internet connection for a while but soon figured out it was not.
Found out I was not alone, but most folks did not have a problem. I couldn't get it to work right so I started preparing Microsoft Edge to use and abandon my old friend.
Decided to try it once more with a fix that was working for some folks on the Mozilla support forum. It worked and the new version performs for me just fine.
Here is the fix: (the firefox menu is the 3 bars near the upper right corner)
Go to the firefox menu [ch8801] > options > privacy & security panel and under permissions check the setting to prevent accessibility services from accessing your browser.
That's all it took.
Tom