First I assume that by "the library ebx.etd files still don't recognize DE" you mean that you are in the process of borrowing a PDF from a Library (which one?) and that clicking on the library's provided download link in the browser (Safari, Firefox or something else?) causes an .etd file to be downloaded, but then nothing happens. Please correct me if I am wrong.
However if that is what is happening:
For a short term workaround while we are trying to figure how to really fix it:
-- Setup --
1.) Launch ADE
2.) Right Click on the icon in the dock and select "Keep in Dock"
-- Workaround --
Drag the .etd file onto the dock icon (shouldn't matter if ADE is running or not).
This will force the connection to ADE - it's not smooth or that user friendly but it should get you reading the books you wanted. ( which was your original goal - not trouble shooting your Mac 🙂 )
If you want to troubleshoot further, the first thing to do will be to check if double clicking on the .etd files after they have been downloaded from the library launches ADE. If they don't, right click on them, select "Open With >" - which should display a list of applications, if ADE is not the default application then something went wrong with the previous steps I sent you (are you sure you clicked the "Change All..." button).
If it does launch ADE, then your browser may be set up to only download files and not launch them afterwards. For instance in the Safari General preferences there is a checkbox - 'Open "safe" files after downloading' - that when left unchecked will cause Safari not to launch anything after downloading - leaving it up to the user to double click on the file after the download is done. So you should check your browser preferences to ensure it will launch files after downloading, if this is what you desire (there are security implications to this however).
Hope this helps.
Jim Lester
Adobe Systems