Please consider turning on send info to Mozilla before posting a question. This helps us help you. As I have no info from your system that would help see things that cause issues. No info is guessing.

Think this will fix your issue. How about uninstall and reinstall Firefox 57 Quantum Full Version Installer/also updates . Site gets it a day before released on Mozilla Servers, used site 20yrs. https://fileforum.betanews.com/detail/Mozilla-Firefox-for-Mac-OS-X/1032985422/3 Note : Legacy Extensions will be disabled and or removed.

Please let us know if this solved your issue or if need further assistance.

Pkshadow said

Please consider turning on send info to Mozilla before posting a question. This helps us help you.

I guess I missed that. My system is macOS 10.13.1 on MacBook Pro.

I have also tried uninstalling and reinstalling. Even tried 57 beta and Nightly 59 version. Nothing works. Problem persists. Just saw it even freezes Finder when I am trying to click 'Get Info' from the context menu in application folder to see info about the Firefox app. So annoying.