They added a new page on the "install add-ons" menu.
on beta1 there were a page "we will add explanation and a nice page here later"
but other add-ons menu are not implemented yet.
for people trying beta2, who still want to use (some of) their 3.x addons :
- Type about:config in the URL bar
- when asked, accept to take the risk
- right-click the main window and add a new boolean : extensions.checkCompatibility.4.0b
- set value to : false
- reboot (one or two time, or sometime you need to boot 3.x first to memorize the active add-ons on 3.x)
I found only DownThemAll not working with 4.x, you need to install
the beta too.
The beta install as a new add-on, disable dTa non beta first.
*currently using 4.0beta3

*