Copy and paste web page text into a text editor and you'll often get the original formatting, which may not be what you want.

Copy Plain Text is a free Firefox add-on which provides the option to copy plain text only.

Once installed, select some formatted text, right-click and you'll see a new "Copy As Plain Text" item. Select this and only plain text will be copied to the clipboard.

If you actually need the formatting, just use "Copy" as before.

That's useful enough, but the extension also has handy reformatting tweaks (it can trim surplus spaces, lines and more), a customisable hotkey, and it can even be made the default Copy action.