Well, it's about to get a little easier. AutoCopy is a Firefox add-on that automatically copies any selected text straight to the Clipboard. (You then paste it like you normally would.)
This probably sounds like no big deal. After all, how hard is it to hit the aforementioned menu or keyboard key?
All I can say is, it's one less step. And once you start using AutoCopy, you'll wonder how you got along without it. Indeed, it has long ranked as one of the handful of Firefox extensions I absolutely positively cannot live without. (Others include PermaTabs and Lazarus.)
Give it a try! I guarantee you won't be disappointed.