When you click on a link in an email or the 'Thunderbird Help', you are expecting the link to open in a browser. The error message suggests that either you do not have a browser application installed on computer which is unlikely OR a browser needs to be set up as default and you need to make sure the computer knows what to open. So, first make sure you have a browser set up as default on computer. then Check associations are set up correctly. In Windows search type: default and select 'Default apps' 'Web browser' - select a browser eg: Firefox Select: 'Choose default applications by file type' All the following should be the default browser eg: Firefox * .htm * .html * .xht * .xhtml Select: 'Choose default applications by protocol' All the following should be the default browser eg: Firefox * HTTP * HTTPS More info: https://support.mozilla.org/en-US/kb/hyperlinks-in-messages-not-workinghttps://support.mozilla.org/en-US/kb/hyperlinks-in-messages-not-working#w_check-that-the-operating-systems-default-browser-is-specified Then check the info as stated at link above for 'Attachments'. The info is a bit out of date as version 78 moved things about. Menu app icon > Options/Preferences > General Scroll down to the 'Files & Attachments' section. If HTTP and/or HTTPS are listed in the 'Content Type' column, either delete the action or ensure the specified action is correct. If you need to locate the 'Config Editor' - it is in the same location - Menu app icon > Options/Preferences > General - scroll to the very bottom.