See the changing the e-mail program used by Firefox article, it gives instructions on how to change the mailto protocol to use Gmail in the "Setting Firefox's mail client" section.

When I use setting in tools /options/application/search : So far so good then a problem : There is no mail or mailto section. What do I do now ?

mailto should be one of the defaults. You can try the following which will reset the file type handling to the defaults.

1. Open the Firefox profile folder, for details of how to find it see the profiles article
2. Close Firefox
3. Rename mimeTypes.rdf to mimeTypes.rdf.old

When you restart Firefox the file type handling will be set back to default values, and mailto should be in the list.

To Tony E, Thank you it worked perfectly. But how do I make G-mail my default on my p.c. it has windows xp, and only gives the options of outlook express or hotmail. Any help would be gratefully received. Cheers

I have never used it myself, but the Gmail Notifier appears to do this. For details see http://labnol.blogspot.com/2006/09/replace-outlook-with-gmail-as-your.html

Still can't use g-mail . When I try sending anything from my p.c. it only tries to use outlook express or hotmail help.

See also http://lifehacker.com/392287/set-firefox-3-to-launch-gmail-for-mailto-links