And here's the error message that shows up in the error console:
Error: uncaught exception: [Exception... "Security error" code: "1000" nsresult: "0x805303e8 (NS_ERROR_DOM_SECURITY_ERR)" location: "file:///export/scratch/reid/flex3.2.0/doc/langref/asdoc.js Line: 493"]...impossible to Google. Bah.
Wandering around the Mozilla bug database a little (using, frankly, a lot of expertise that most people don't have) revealed that, in fact, this is by design.
You can turn off the behavior by going to about:config and setting the secret configuration variable security.fileuri.strict_origin_policy to false. Mozilla developers are not interested in making this more obvious and don't believe that anyone other than a web developer needs to change it, despite the large installed base of on-disk HTML user manuals for a variety of things.
I don't fully understand the security reasons for this change. I assume they're sound. But what an absurdly opaque failure mode.