Nevermind. I figured-out the solution myself. It's equivalently as dangerous as editing the Windows registry; if anyone wants to try this, I would recommend to first back-up the prefs.js file in the Users or "Documents and Settings" folder ('''not '''the one in the Mozilla Firefox folder). And you may have to enable "view hidden files/folders" in the Windows Folder Options to be able to do so. Before starting, you must find what URL (with "?<query>{&<additional queries>}" appendants) is used by your email service for starting a new email from the browser address line (see my initial question for an example of Hotmail's). This may require you to compose a temporary new email to see what URL is used by your email service (unfortunately, this often does not give you the right URL). Or, you have to do some research in the Internet like I did. Don't forget to test it too. After doing the above, type "about:config" in the URL line of Firefox, and click "I'll be careful, I promise" button in the message that pops up. Then type "gecko" in the filter line (just under the tabs). You want to look for a pair of Preferences with "mailto.<number>.name" and "mailto.<number>.uriTemplate" in the names respectively. Currently, the <number> is numbered 0-3 with the first two already set with defaults. I put mine in pair #2, but I suspect you can replace one of the first two defaults as well. (Note, user defined preferences are in bold, and even says so in the "Status" column.) Right-click the mailto.<number>.name line, and select "Modify." Replace what is there with the name of the email service you want to use, and click OK. Next, right-click the mailto.<number>.uriTemplate line, and select Modify. Again replace what is there with the URL (and ?<query> etc) to use. Be sure to replace (in your URL) what would be the recipient's email address with "%s" (without quotes), and click OK (again, see my Hotmail example). One last thing before you can close the about:config tab/window. Firefox does not bother to read your new settings unless you put a/any new number in the "gecko.handlerService.defaultHandlersVersion" line, which should currently be "3". I simply added ".1" to the end of that number. Right-click the line, put in your new number, and click OK. Now you can close the window; probably best to shut-down and restart Firefox. Now you are all done. ----- P.S. Why did I not just use the add-ons/extensions? Well, it is often not clear what those things would add to Firefox: a preference update, or something else. Also, the ones I came across did more than just Hotmail. Since Firefox already came with Yahoo and Gmail, why get an add-on that does the same thing? In the future, I may eventually change to an add-on if (as some have claimed now) Hotmail/Microsoft does starts to constantly change that new-email-URL.