Many extensions aren’t compatible with firefox 3 and then are disabled. But most of them would work.
Here’s a little workaround for this issue.
WARNING : By doing this, you may have to deal with extensions bugs and maybe security holes !
In the firefox address bar type :
about:config
then press the “I’ll be careful, I promise” button.
Now right click anywhere in the window. Pick New > Boolean in the popup menu.
Type this name in the dialog box :
extensions.checkCompatibility
and in the next dialog, choose the value false
Now, create another boolean key with the right click menu with this name :
extensions.checkUpdateSecurity
and, again set the value to false
Restart firefox.
Have fun.

english
français
No, this will not make firebug installable and usable if you use the firebug xpi from mozilla addons mirrors.
Use the firebug beta version instead. It’s “instable” but works correctly here.
Get the beta from firebug website : http://www.getfirebug.com/
edit: oups, looks like their site is down today :-/