You can easily make your add-ons work with the new beta. What you need to do is find the Firefox folder in your AppData folder (Vista will be in C:\Userdata\[yourname]\AppData\Roaming\Mozilla\Firefox\Profiles\ , XP should be C:\Documents and Settings\[yourname]\AppData\Mozilla\Firefox\Profiles) Click on the folder extensions, then go through the folders and open the file "install.rdf" with notepad/wordpad/etc. Find the line that says something like : 0.8
and replace the maxVersion data with '3.*.*'. Now that extension will work with all versions of Firefox 3, beta or not.
One note though, this just makes Firefox think the extension is compatible. 90% of the time, they will be, since not much code is changed with these small updates. If an extension isn't actually compatible and you do this, firefox will likely crash and not run.