Go to your computers Programs Folder, and remove Java. Does that fix your issue? For now, don't worry about not having it.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the "Applications" folder. Linux: Check your user manual.

I don't know whether you noticed, but the Java installer pushes an add-on from Ask. If you didn't reject that, it's possible the new extension is causing issues.

Could you try starting Firefox in its Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: You can start Firefox in its Safe Mode by holding down the Shift key while double-clicking your Firefox shortcut.

A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Will Firefox start?