Follow these steps to completely remove and reinstall Firefox. This process will not remove your Firefox profile data (such as bookmarks and passwords),

   Download and save the installer for the latest official version of Firefox from mozilla.org.
   Exit Firefox (if open): Click the Firefox menu Fx57Menu and select Exit.
   Uninstall Firefox from your computer.
   Delete the Firefox program folder, which is located in one of these locations by default:
       C:\Program Files\Mozilla Firefox
       C:\Program Files (x86)\Mozilla Firefox 
   Reinstall Firefox using the installer you downloaded earlier - see How to download and install Firefox on Windows for instructions. 

Now start Firefox and check to see if your problem has been fixed. If it hasn't, continue with the next troubleshooting method.

And for refreshing Firefox follow these steps

   Click this Refresh Firefox button directly, if you are viewing this page in Firefox. This won't work if you are using a different browser or a mobile device.
       You can also find a Refresh Firefox button at the top of the Firefox about:support Troubleshooting Information page. 
   To continue, click Refresh Firefox in the confirmation window that opens.
   Firefox will close to refresh itself. When finished, a window will list your imported information. Click Finish and Firefox will open.

If you want to reinstall Firefox, follow the instructions below. If you just want to remove it, disregard the install instructions.

Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia