It's impossible to set Gmail as default email client. But this can be fixed with an extension.
Just try to click on any email adress on nay website... It opens some of any default desktop email client that you have on your system. It's impossible to set Gmail as default email client. But this can be fixed with an extension.
This is autotranslated version of this post.
Mailto: Set Default Email to Gmail by cloudHQ will open new email form in Gmail when you click on a mailto: link.
You just need to install the extension and the magic will do everything for you.
Install | Alternative to Firefox (G)mailto: