I have these entries in my config editor:

network.protocol-handler.app.http;/opt/firefox/firefox network.protocol-handler.app.https;/opt/firefox/firefox network.protocol-handler.expose-all;true

…where /opt/firefox/firefox is the path to the firefox executable on this Linux system. You need to substitute the correct path used by your machine.

I'm not sure why these are set. They are in bold type, showing that they have been changed from the default setting by the user (i.e. me). If you don't have these settings, then Thunderbird ought to just use the default browser.

First check you need to do is to make sure that Firefox is the default browser for your OS. If it isn't, could it be made to be? If not, you may have to use those config editor settings to declare the browser to be called by Thunderbird.

Thanks for the response but sorry I forgot to say tha I am using Windows Vista home Premium service pack 2.

your response is for Linux.

And I said "your machine" and "your operating system" because the details need to be adapted to whatever platform is in use. I've shown you the network handler options you can set to call whatever browser you want to have run for you. I don't know precisely where on your machine Firefox is to be found; that part of the job is for you to do.