Like many other Firefox users, I recently upgraded to Firefox 3.  Unfortunately, when I went to install Firebug 1.2, I ran into this error:

Firefox could not install the file at
https://addons.mozilla.org/en-US/firefox/downloads/file/30423/firebug-1.2.0b3-fx.xpi
because: Unexpected installation error
Review the Error Console log for more details.
-203

The Firefox error console showed this:

Error: installLocation has no properties
Source File: file:///opt/firefox/components/nsExtensionManager.js
Line: 3849

After a bit of headscratching, I was finally able to resolve this issue by closing Firefox, deleting the following files from my profile (how to find your Firefox profile), then restarting Firefox:

• extensions.ini
• extensions.cache
• extensions.rdf

I am still not entirely sure what the problem was, but that seemed to fix it.  Your mileage may vary.  Just thought I'd post this in case anyone else is having the same problem.