I'm not able to Upload My files using Firefox. Why Not?

Try "Reseting Firefox to its default state". 

That should fix this issue.

1. Click the menu button , click Help and select More Troubleshooting Information.
2. Click Refresh Firefox then Refresh Firefox in the confirmation window that opens. Firefox will close to refresh itself.
3. When finished, a window will list your imported information. Click the Finish button. Firefox will open.
4. Select whether you want Firefox to restore all or some windows and tabs and click the Let's go! button.

This may remove your add ons and other customizations such as extension, etc. You may need to add them back after refreshing.