If the "Use Adobe Reader (in Firefox)" action is not presented, it suggests that the pre-requisite plugin is not installed. See my answer to another similar question for further information, including how to determine whether the plugin is present and active in Firefox.

At the filesystem level, installing Adobe Reader should result in a file named "nppdf32.dll" appearing under the "C:\Program Files\Mozilla Firefox\plugins" folder.

The plugin is installed correctly. The problem is not that I can't use adobe to open pdfs in firefox--it works fine for "PDF (adobe)" (or whatever the listing is), but if there's a mime type problem or error (I assume from when the document was originally uploaded), then I get some pdf files that list as "PDF (application/unknown)." For whatever reason, the adobe plug-in doesn't associate with "PDF (application/unknown)" files.

I see. Can you provide an URL where it does not open through the plugin as expected? HTTP (web) servers are responsible for asserting the MIME type of the content in reponse to the user-agent (browser). If the server does not define the MIME type as being "application/pdf" before proceeding to serve the content then the plug-in will not kick in. That's one possible tangent.

The other is that something is wrong in Firefox. In that case, try resetting the download actions.