Always take care when changing the registry.
I use Windows 7. How do I set up my gmail as my default email client?
Backup registry before making changes you aren't sure of.
1. Open regedit and navigate to: HKEY_LOCAL_MACHINE\SOFTWARE\Classes\mailto\shell\open\command
2. Double-click the '(Default)' value name
3. Insert this line: "firefox" https://mail.google.com/mail?extsrc=mailto&url=%1
If you use chrome just replace "firefox" with "chrome". Haven't tried with other browsers, but they should follow similar pattern.