Yes, that's correct. They are restartless extensions (can be installed or uninstalled without restarting the browser) and that is only supported starting with Firefox 4 (or even Firefox 8 if you look at all the other features required). Next Adblock Plus release will require at least Firefox 8 as well.
Please update to a current Firefox version, like Firefox 12 - Firefox 3.6 is no longer safe. It isn't supported by Mozilla so security issues in that browser are no longer being fixed.