pbw, you can now find the Mailto button extension in the extension catalog. It will add a context-menu entry when you right-click on a website and a toolbar button. The toolbar button can be hidden from the extension manager if you do not want it.
This may not even be a problem in Opera. I have been able to log in to Blogger, Gmail and GMaps accounts, logging out in between, and being offered a n empty login screen each time. My move to Opera may be exactly the right thing.