Confronted with this issue I found that I need to run a version of Chromium that is newer then the one in the repositories in order to use an extension that I want.
Can I get a such new, even latest version?
(At the present moment Chromium is 20.01, and the X-notifier extension needs 21.x. So it works ok in Google Chrome which is v. 23.0.)
In Windows I can use beta Chromium portable that is newer than Google Chrome.
How to get that newest Chromium here in Ubuntu?