I'm looking for browser extension that can quickly copy page source code to clipboard. (For example, just click button and all copied).
Is next combination is not quick? (considering you are in FF)
Ctrl +U open source code pop-up
Ctrl + A select all
Ctrl + C copy to clipboard
Ctrl + W close source code pop-up
Also look at this plugin
This extension doesn't copy the real source code but it download the page in a single file with the source code modified a little bit (hehe)...