Is it this issue:

Some users are finding that an update changed the description of "Adobe Acrobat Document" to "Firefox HTML Document". The installer was supposed to add Firefox as an ADDITIONAL viewer for PDFs, not as the DEFAULT viewer. Sorry if you were affected by this glitch and hopefully they will figure out why some systems get changed this way.

You can try this fix suggested by a user in another thread:

1. Open Adobe Reader / Acrobat*
2. Edit->Preferences
3. In the Categories column click 'General'
4. Near the bottom of the page click the button marked 'Select Default PDF Handler'
5. In the dialog, select 'Adobe Reader XI' (or Adobe Acrobat, as the case may be) and click 'Apply'
6. A Windows Configuration screen will appear. Allow it to do its stuff (takes a few minutes), then restart your computer when prompted.

Does that work for you?

* If you do not have Adobe Reader 11, you can install it from here: http://get.adobe.com/reader/

Many thanks. Much appreciated.

Your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts.

The user.js file is only present if you or other software has created this file and normally it wouldn't be there. You can check its content with a plain text editor (right-click: Open with) if you didn't create this file yourself.

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

See also:

• http://kb.mozillazine.org/Preferences_not_saved