Since my browser was changed to "Firefox" I am having frequent "timeouts" and error messages indicating errors in addresses;

Do you know what might be causing this?

No. I don't know what's causing it, but I would recommend you uninstall and reinstall Firefox. There should be no reason that Firefox won't work properly if Internet Explorer is working properly.

Download CCleaner (CCleaner - Home) and run the cleaner and the registry cleaner (run the registry portion several times) after you uninstall Firefox and before reinstalling it. This will ensure that everything is straight before you reinstall it.