There seem to be several settings with Thunderbird that can change how web URLs contained in emails will be opened on a user's system. Finding and altering these settings is difficult for your users. And it should not be necessary. I previously thought I had this problem solved by ensuring my "network.protocol-handler.app.http(s)" preferences were set to use Firefox. Then a few days ago, I installed Google Chrome on my Fedora 22 system. Suddenly my web links within emails were opening with Chrome. A search of my Thunderbird preferences revealed no references to Google Chrome anywhere. A search of my KDE preferences revealed no references to Google Chrome as the default browser. I see NOW that this is wrapped up somehow with -attachments- (guys, links are not attachments) within Thunderbird and I swear that I checked those settings and could not find the reference there either until I reset my "network.protocol-handler.warn-external.http" preference as described here: http://askubuntu.com/questions/130158/how-do-i-make-thunderbird-open-links-in-chromium. The only "official" article on this matter I could find: http://kb.mozillazine.org/Changing_the_web_browser_invoked_by_Thunderbird is hopelessly out of date and insufficient considering the number of knobs and dials that have been added to alter this setting in the intervening years. At a minimum, I would ask you to update the knowledge base with a more complete answer. A more optimal solution would be to improve the user experience to control these settings with greater certainty.