Firefox just won’t open any more
QWhenever I open Firefox 48.0.2, it quits immediately. Is this caused by a bug in Firefox?
APerhaps, but there are other possible causes you should rule out. The first is an old and incompatible extension, in either /Library/Application Support/Mozilla/ Extensions or the same path in your Home folder; in Finder, hold å and pick Go > Library. Any which look old or unwanted should be moved to a folder in Documents, which may allow Firefox to start up normally again.
In a new Finder window, locate Firefox, then hold å and double-click the app’s icon to open the browser’s Safe Mode. That mode eliminates third-party extensions, themes and accelerators you’ve added to the browser. Mozilla provides info about troubleshooting in Safe Mode at mzl.la/2f7zu8M.
Another potential solution is to remove Firefox’s preferences file, which is located in the Library folder within your Home folder. There, open the Preferences folder and trash the file called org.mozilla.firefox.plist. If none of these steps resolves your problem, you may find Firefox’s next version runs better.
Firefox's Safe Mode disables browser extensions, and lets it check extensions for compatibility and other problems.
Firefox extensions are in Library at the top of your startup disk and in your Home folder. Remove unwanted items.