I had to hack OpenDownload, remove Fasterfox, Greasemonkey (doesn't support RC1 yet) and Broadband Speed Test and Diagnostics.
EDIT: I hacked the Broadband one too
I got the info off the OpenDownload page, someone posted a comment:
An invaluable add-on that is currently broken for Firefox beta 3, but here is how to fix it to work! 1. Register and login to addons.mozilla.org. This will bring up a link "Ignore version check". Click that then right click "Add to Firefox" to save the extension to your desktop. 2. Rename opendownload-1.0.0-fx.xpi to opendownload-1.0.0-fx.zip, then open in WinZip or similar program 3. Open the install.rdf in WordPad or similar text editor (Notepad doesn't work well) 4. Near the bottom, modify the line: 2.0.0.* to read: 3.0.0.* Save and close install.rdf 5. If using WinZip, it will detect install.rdf has been changed and will prompt you to update the archive. Click Yes. 6. Close WinZip, and rename .zip file back to .xpi You should then be able to install plug-in as normal (just drag-and-drop the file into your Firefox window) Hope this helps!