Can someone tell me what this means

does it help if you download a fresh firefox from https://www.mozilla.org/en-US/firefox/all/, reboot your system once and then run the setup and install firefox again?

Firefox is my only browser can you give me an idea of how to download a fresh version

you appear to be on windows 7 - presumably there's still internet explorer lying around somewhere on your system. you can press the windows-key + R and open "iexplore https://www.mozilla.org/en-US/firefox/all/" in order to get to the firefox download page with IE...