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.