The days of Microsoft Outlook being the de facto email client for the masses are gone. With web-based email clients such as yahoo, msn and gmail being the most prevalent email clients in our day, one might ask, "How can I get my gmail client to be the default client for my web browser?" I don't know the answer to all the different clients, but I have been able to find out how to make gmail your default client. Below are instructions for each browser:
Visit this nifty addon called Mailto:Gmail, click install and restart chrome.
Go to the FireFox menu in the top left. Click on Options. Click on the "Applications" Tab. Search for "mailto" in the search box, and select "Use Gmail" from the Actions drop down list.
Internet Explorer 9 (IE 9)
Sorry, but <sarcasm>there's no reason that you shouldn't be using Outlook as your email client</sarcasm>, so you'll have to download a little app called Gmail Notifier for Windows. Therein you will be able to handle your default mail client for each browser.
Sorry, Safari users. I don't have an answer for you, other than that you should be using Chrome or Firefox.