AutoCopy

Select text and it's automatically copied to the clipboard. Also pastes on middle click.

In addition:
Developer: Is this spyware? by stevie10999 on March 7, 2014

"Developer needs to confirm immediately if this addon is sending out ANY info.

Been using this addon for a long time, and just looked at source. Wips.com is scattered throughtout the sources in the javascript files."

One might wish to try AutoCopy 2 by Leszek Życzkowski.