Unable to open default home page in FireFox

This behavior can sometimes be caused by an add-on installed in Firefox. To figure out if this might be the cause, start Firefox in Safe Mode which will cause Firefox to load without any add-ons running. To start Firefox in Safe Mode, hold the Shift key while clicking on the Firefox icon and you will be prompted to start in Safe Mode or Reset Firefox. Choose Safe Mode and see if Firefox loads up the default page you are trying to set. If it does, then one of your add-ons or custom settings is causing the problem and you will have to troubleshoot that by disabling and re-enabling add-ons till you figure out which one is causing the problem. For more information have a look at the Firefox support article called Troubleshoot and diagnose Firefox problems. Alternately, you can go back through the process I described to start Firefox in Safe Mode and choose the Reset option. This will reset Firefox back to the way it was when first installed with no add-on and no customizations. Once you do that Firefox should work as expected and you can then go back and setup your customizations and reinstall any add-ons and plugins you were using. And not to worry, resetting Firefox will not delete your bookmarks.