Recently whenever Firefox starts it doesn't show my Home Page but instead it's a completely blank page witha tab heading 'New Tab'. The Home Page opens as normal if I click on the icon in the toolbar or if I open it from bookmarks. A similar thing happens on when opening a link - but not always. I've no idea which links they are but sometimes they open as normal and sometimes just a blank page with 'New Tab' in the tab title. I'm using the add-on 'Tab Mix Plus' (and have done for many previous versions of Firefox) but changing settings there makes no difference. It seems to have started when v56 came out.