An addon for Firefox that will automatically paste stuff into Firefox. (just grab the .xpi file and open it with Firefox to install)
To use, click the icon to turn it on, refresh the tab you want stuff to get pasted into, then click the icon to turn it off (unless you want stuff pasted into your other tabs as well). It will keep running in that tab until you close it or refresh it.
It tires to put the text into the first textarea it finds on the page. I tested it on http://isoron.org/stuff/japanese/echo.html and google translate, it may or may not work for other sites.
Oh, and I'm using Firefox 29 on windows 8, I have not tested it in any other environments.