If you go into Manage Applications and tap the current default application, you can clear the default status. (At least on Gingerbread.)

Association list is empty for both FF and selected default application

Probably FF manage "mime type -> application" association list somewhere, but I can't find such option in FF settings.

I'm not familiar with an Association list. What I know might be obsolete, but below the memory usage information you should find a section called Launch by default where you can clear the program from being the default application. If you like pictures: http://blog.laptopmag.com/how-to-change-default-apps-in-android

The dialog you clicked created an Android system setting. You will need to check the app you set as a default.

This support article will get you into the right area https://support.mozilla.org/en-US/kb/make-firefox-default-browser-android though you will need to figure out what app you set as the default.

jscher2000 : by association list I mean Launch by default section. And its empty for Firefox application and for nTorrent application which is now bound to links to .torrent files

kbrosnan: You're right, its the place where I tried to find this association. But it is hard to find out an application which "Launch by default" settings should be cleaned up.

Go to about:config (yes, it works on mobile).

Here search for browser.download.preferred.aplication and you'll find the offending preferences. You can use the "reset" button to remove them.

This caused an enormous amount of grief for me and I found the solution by chance.

Terrible workarounds include completely deleting Firefox's data, and uninstalling the application that was selected.

@amelape: It is strange from my point of view that Firefox developers decided to avoid standard Android mechanisms (clear application data), but about:config way works. Thanks a lot!