I figured out a way to use many extensions and addons with Firefox Beta (note that it does not work with skins, since they are .jar files).
1. Go to the download page for your extension of choice.
2. Right click on the download link and save as to a location of your choice. It should be a .xpi file.
3. Open the .xpi file (with WinRAR preferably).
4. Locate the file "install.rdf". It is not inside any sub folders and should be alongside a .xul file.
5. Inside install.rdf, look for a tag that says "<!-- Firefox -->". Just a note for those of you who use SciTE, it will be in green.
6. Under the Firefox comment, look for a tag called "<maxVersion>".
7. The number next to <maxVersion> will likely be 2.0.0.*. Change this number to 3.0.0.*. (those last periods are the ends of sentences--do not include them)
8. Save "install.rdf". You can close WinRAR now.
9. Find the file where you saved it and drag it into your Firefox icon. This will cause it to install.
NOTE: This does not work with all extensions, as some of them are physically incompatible with Firefox 3 Beta 3. Notable extensions I've tried that do not work are Download Statusbar and Firebug. Some Greasemonkey extensions will not work properly (the Homestar Runner All-in-One will not work correctly, though I wrote a script for GameFAQs that works correctly still). Extensions like Web Developer, Fasterfox, and Javascript Debugger still work.