This was my prototype for my official article on the Mozilla site. This post has not been updated for Thunderbird 5, the one at Mozilla has.
Each operating system has a 'default browser' for viewing web pages. Internet Explorer and Firefox, as well as others I assume, check this setting by default when they start up and prompt the you to set them as the default. Sometime this process goes a little awry for some reason or another and your computer looses the details of the default browser. To cure this problem you simply need to get the browser of your choice to check this setting and update it.
In FireFox the setting can be found by going to the Tools menu and selecting options. Select the advanced picture and then click the check now button. Unless you regularly use multiple web browsers it is probably a good idea to check the box on the right to have Firefox check each time it starts.
Internet Explorer under XP (it may differ on other windows versions)
Go to the control Panel select the internet options. Select the programs tab and in the middle of the page will be options to turn the checking on and off.
Ok so that did not work
It is time to look for an extension that may be causing some unwanted behavior. Run Thunderbird in Safe Mode. There is an entry on the Help menu for Thunderbird (Start with add-ons disabled) that you can use. Safe mode disables extensions and allows you to check if the unwanted behavior is cause by one of them.
Next is to try the Standard Diagnostics.
If you still have a problem, it is time to go to Get Satisfaction and ask about what could be the cause of the problem.
Before you post in Get Satisfaction read this and be sure to say exactly what you have done already to diagnose the problem.