There is a way around that... I am trying out the Firefox 3 Beta 4. If you feel comfortable in about:config... try this...
in a new tab, type in about:config. Now in the page that appears, right click, select new, then click on Boolean.
In the box type in "extensions.checkUpdateSecurity" and click on ok. In the next box, set the preference to false. Now restart firefox. You'll have a warning in the addon manager under extensions warning you that add on update security is disabled, but everything should work like you are wanting it to.
Also another thing to try to see if your extensions work with this version of Firefox is to search for "extensions.checkCompatibility" and set that to false as well. This COULD cause problems though, if your extension you are wanting to use is not actually compatible with Firefox 3. Hope this helps.