Is there a way to force Firefox 3.0 beta to load plugins that don't claim to support that version of the browser?

Yeah, you can download the xpi locally and change the extension to .zip

Open that like you would with a normal zip archive. So now you have a folder. Inside that folder find the file called install.rdf and open that in a text editor. Don’t use Word, use something like Notepad.

In that file you will find the line:
<em:maxVersion>2.0.0.*</em:maxVersion>

Change that to:
<em:maxVersion>3.0.0.*</em:maxVersion>

Save that and zip that back up. Change the zipped files extension back to a .xpi

Then you can double click that and firefox should install it.

Keep in mind that doing this is not recommended and could break stuff.

I would use johnpowell’s suggestion to install a copy of MR Tech Local Install xpi, it has a right click feature that lets you force-enable incompatible extensions.

Check out the Nightly Tester Tools extension (http://www.oxymoronical.com/web/firefox/nightly).

If you have an extension installed in Firefox2 you can force it to be compatible with Firefox3.