Hi, try clearing cache and cookies.

under Permissions check (put a check mark) in the setting to Prevent Accessibility Services from accessing your browser.

Do a scan : https://www.malwarebytes.com/mac/

You can also check for issues with the sessionstore files in the sessionstore-backups folder folder.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox