The following is how to set GMail as your default email client when you click on email addresses from your browser.
Chrome: How to Set Gmail as Your Default Email Client
Step 1: Open Gmail in Chrome and click the Protocol Handler icon in your browser's address bar. If you don't see this icon, skip ahead to step 3.
Step 2: When prompted to "Allow Gmail to open all email links?", select "Use Gmail," and click "Done." Then you're done! Skip steps 3-5.
Step 3: If you were unable to see the Protocol Handler icon , click the Chrome menu icon in the top right corner of your browser, and choose "Settings."
Step 4: Click the "Show Advanced Settings" link at the bottom of the screen. Then click the "Content Settings" button under the "Privacy" header.
Step 5: In the pop-up window, scroll down to the "Handlers" section and click the "Manage Handlers" button. Then select Gmail from the mailto dropdown, click "Done," and, well, you're done!
Note: If you're still having trouble (for example, your "Protocol handlers" window appears blank with no options), try installing Google's Mailto Extension for Chrome instead.
Firefox: How to Set Gmail as Your Default Email Client
Step 1: In the top left corner of your Firefox browser, click the Firefox menu button. Then choose "Options."
Step 2: Click the "Applications" tab, type "mailto" into the search bar, and click the drop-down arrow next to the "Action" column. Select "Use Gmail," click "OK," and you're done!
Internet Explorer: How to Set Gmail as Your Default Email Client
Step 2: Once installed, click the wrench icon at the top right corner of your Internet Explorer browser to open the Toolbar Options window. Scroll down to the "Web-browsing tools" section, select "Use Gmail for 'Mail To' links," and click "Save." Now you're good to go!
Safari: How to Set Gmail as Your Default Email Client
Step 1: Click here to download Google's Mailto Extension for Safari. Click the arrow at the top right of your Safari browser, click the download, and click "Install" in the pop-up window.
Step 2: Select Gmail from the list of options.
Step 3: Restart your browser, and you're all set!