If you run Firefox Browser you may know of the 'Get Mail' add-on from Web Design.
Although the 'Read Mail button works the 'New Message' button doesn't, the cure is simple -:
Click Firefox 'Tools', then 'Options' down at the bottom, then click 'Applications'.
In applications look for 'Mail To', open the drop down window on the right hand side and select 'Use other', then click browse to search and select the Thunderbird Icon... click OK and your done. You will find the 'New Message' button will now open the new message page ready for you to type.
The Default setting you'll see Use mailto (Default) can be misleading and you can be forgiven in thinking this is the default browser...
What you should see is the Thunderbird Icon and Use Mozilla Thunderbird for the button to work.
Fix 'Get Mail' in Firefox 3 
Linear Mode
