Normally, clicking on an e-mail address on a Web page will open up a new e-mail using your default e-mail program. If you'd rather use Yahoo Mail, Click on the 'Tools' menu and select 'Options'. Next, select the 'Applications tab' and scroll down to the 'mailto:' entry. Next, select 'Use Yahoo Mail' and click 'OK'.
Gmail is not included as a built-in option in Firefox, but you can add Gmail easily enough. Type 'about:config' in the address bar and hit Enter. In the 'Filter' field, type 'gecko'. Double-click on the 'gecko.handlerServiceAllowRegisterFromDifferentHost' entry that appears below. This will change the value from False to True. Next, cut-and-paste this line into the address bar and hit Enter:
A message will appear at the top of the browser window asking if you want to add Gmail as an application. Now, repeat the process above for choosing Yahoo Mail, but select the new Use Gmail option instead.