This is an OKAY extension, quite a bit old.

Good thing is there are better alternatives now. "CopyIn2Clicks" being one of them!

Basic functionality works well. It copies text that you select.
The problem with this (and every other auto-copy webextension I've found) is recognizing which fields *not* to copy from. For example, this extension doesn't copy from basic text fields, which is good. But it does copy from Gmail's compose field, which is bad.
A simple mitigation for this would be a user-manageable site blacklist. Alas, this extension does not have that, or any options at all.
My search for an auto-copy extension that really works right must continue ...

its ok, but the i really MUST to have the feature to deselect text after copying. Paste with middle click is good to have as well.

Doesn't work on addons.mozilla.org website. (why?)