I can't get access to Firefox, my default browser.When I click on the Firefox icon, a pop-up message appears. 'Your Firefox profile cannot be loaded. It may be missing or inaccessible.'

This has been the case for three days now. I've tried system restore repeatedly - no change. I've tried re-installing Firefox. Apparently it goes through ok, but then, when I try to open it, up comes that message again. I've tried uninstalling AGV security, in case that is what is blocking access, and then re-installing Firefox. Same result. This is really bugging me. Can anyone help?