Not a FF AddOn per se, but I've added support for Firefox in Transpose hotkey utility. It allows me to uninstall AutoCopy AddOn.
One thing annoying about auto copy extensions is when you want to select text then use a button like insert link the auto copy messes you up by copying the selected text to the clipboard, making it just about impossible to paste a link after pressing the Insert Link button on a forum.
The copy text to clipboard in Transpose only copies to clipboard if you select text with the mouse, then press the Alt key and release the key and the mouse button. The text will deselect and you'll know it's in the clipboard.
It also works in Chromium and Opera browsers. IE doesn't like it, probably due to the mouse button action. But you could probably get it to work with a different hotkey. I did the Alt-LeftMouseButtonUP combination because it doesn't interfere with standard right mouse click context menu then press Copy action. Also if you select text with the keyboard it doesn't come into play at all, making it easy to use Insert Link buttons etc..
You can see latest details here:https://www.donation....msg249292#msg249292