Thunderbird will open and display pages from Thunderbird.net as a part of it's operation, but pages from non Thunderbird.net sites are not offered the privileged loading that Thunderbird.net get and those pages are loaded in your browser.

Thanks for your answer but I don't think this is exactly the problem. Let me try and explain further. Under Thunderbird options then general then when Thunderbird launches show the start page in the message area I have specified an external webpage. This webpage displays correctly as expected within Thunderbird. It contains information which changes every day. It has links to display this information in more detail. Clicking one of these links brings up the What should Thunderbird do with this file? pop-up box. Thunderbird knows the file name, that it is an HTML Chrome document and the web address. The Chrome browser now opens correctly but displays the page from my PC hard drive rather than opening it from the internet. The file stored on my PC is not the latest version of the webpage required. Is this a bug in Thunderbird? Can this be changed somewhere in Thunderbird settings?