Some webpages contain email links. If you right-click on the link in most Web browsers, a menu appears that lets you copy the email address to the clipboard (first screenshot). You can then paste the address into the To field of a new email message.
In recent versions of Mozilla's Firefox browser, you can also left-click on the link and get some action. If Mozilla's Thunderbird is your default mail program, a Thunderbird 'compose' window may appear with the To field automatically filled in. This article explains how you can get the same automatic result under Linux with the excellent open-source mail programs Sylpheed and Claws Mail. The method also works with Iceweasel, which is the rebranded Firefox packaged with Debian GNU/Linux.
A one-command script
First, copy the script below into a text editor and save the file as mailto.sh. If Claws Mail is your mail program, replace 'sylpheed' in the script with 'claws-mail'.
sylpheed --compose "$1"
Put the file someplace handy in your home folder. I keep all my homemade scripts in a folder called 'scripts' (/home/bob/scripts), which is very easy to find and manage. (More sophisticated Linux users might want to put the script in one of their $PATH default locations, such as /usr/local/bin, or in a hidden home folder, such as /home/[username]/.scripts.)
Make the script executable by right-clicking on the file mailto.sh in your file manager and looking under Properties for the option to run the file as a program. Alternatively, navigate to your scripts folder in a terminal and enter
chmod +x mailto.sh.
Set up your browser
You now have to tell Firefox what you've done. Go to the Applications tab in the Preferences dialog (second screenshot). When you select 'mailto' in the Content Type column on the left, you'll see a drop-down list in the Action column on the right. I don't have Thunderbird on my computer, and in my version of Iceweasel, the choices on the drop-down list are Always ask, Use Gmail, Use Yahoo! Mail and Use other...
Choose Use other... and a window opens in which you can select a helper application (third screenshot). Navigate in this window to the folder where you put mailto.sh, select mailto.sh and choose Open.
All finished. Next time you left-click on an email link, Sylpheed (or Claws Mail) will open a compose-message window over the webpage, with the email address in the To field (last screenshot).