for chromium and firefox? I've changed them in menu editor, but when launched it appears the same standard icon on the panel. Is there a way to change them?

Unfortunately you can't change those, as they are set by the application themselves. If you can alter Firefox/Chromium to provide these different icons to X, then it will work.

Not true. I managed to change firefox icons by changing the default ones in /usr/lib/firefox...
but I cannot find chromium default icon....

It is probable that the Firefox executable loads that icon from disk in order to send it to the X server.... I'd expect Chromium to do something similar to that, i'd suggest looking under /usr/share.