Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
What version of Mac OS X are you using ? You can find it from Apple menu > About This Mac
Download a fresh copy of Firefox from here --> https://download.mozilla.org/?product=firefox-46.0.1-SSL&os=osx&lang=en-US
Open the downloaded Firefox file. Then click and drag the downloaded Firefox app to your Applications folder. If asked to replace older version, then click Replace or Yes.
This will install a fresh Firefox. If you have any difficulty, please let me know and I will continue to help you