Firefox 3 could not install firebug 1.2

Here is the error message I got when I tried to install firebug 1.2 in Firefox 3.

Firefox could not install the file at

https://addons.mozilla.org/en-US/firefox/downloads/file/30423/firebug-1.2.0b3-fx.xpi

because: Unexpected installation error
Review the Error Console log for more details.
-203

firefox 3 faster

Two main things that you wanted in a browser were speed and good memory management.
Firefox 3 has them, as far as I have seen.
Happy browsing with firefox 3, 'the best firefox yet'.

One of the other things that firefox can look into is 'text and image rendering', IE (Internet Explorer) seems to be better at that still. Things look more pleasant in IE than firefox. In this aspect even Opera is better than firefox.

why both interface and an abstract class

An interface can have no implementation.

An abstract class can have some implementation as well as abstract methods which doesn't have implementation.

We have abstract classes to have the benefit of both the interface and a class.

In an abstract class, functionalities common for a module can be implemented in the methods, at the same time the methods that must be implemented by the classes specific for an operation can also be specified/enforced.

So an abstract class is an additional facility rather than a necessity, a nice to have feature in any OO language like Java.