In cooperation with the userscript: "accesskeys-reveal" ( http://userscripts-mirror.org/scripts/show/8376 ) solves impossibility to add CSS-generated content after form elements, so this one will show you accesskeys of any input and textarea.

Try / test it here:
http://eldar.cz/myf/pub/firefox/accesskeys-test-area.html

Changelog:
2007-11-15 : screen shots (taken from http://kyberia.sk/ )
2007-10-14 + few bugs fixed, code cleanup. (Also update the userscript!)
2007-10-11 + opacity: 0.8;
2012-06-15 docs, select, unprefixed for modern Fx. update userscript.
2012-06-18 minor fix for pages already altering generated contents' appearance