"…maybe Adobe Help uses Opera for the browser."
That could be the case for you, since you apparently have Opera installed. But it wouldn’t be the case for everyone, sooo…
Perhaps ditching the .plist file for the browser you prefer, Steven, will work for you. Perhaps there’s some sort of disconnect between Photoshop and the way it tells the browser to open the help files.
First, though, navigate to the folder within the Main Photoshop folder that contains the HTML help files.
Highlight one of the files (e.g. "help.html") in a Finder window. Press Command + I to Get Info. In the window that appears, scan down to the "Open With…" pane and select the browser you prefer from the drop down menu. If you select something different from what appears there when you first summon the "Get Info" window, you’ll also be able to choose "Select All" to change the default application for all HTML documents to the browser of your choice.
If this doesn’t fix it, have a go at repairing permissions and re-prebinding files & applications using the free Onyx maintenance utility <
http://www.titanium.free.fr/pgs/english.html>, Cocktail, or what have you. (Re-prebinding isn’t supposed to be necessary under newer versions of OS X, but it can’t hurt.) Another thing that may help is to run the Launch Services rebuild routine built into many of the up-to-date maintenance utilities.
I’ve created a Bookmark in my browser of choice (Firefox, natch :)) and access the HTML help files from there. I’ve never had a problem, for what it’s worth. I just checked, and Firefox loads the main frameset of the Help files if I click on the Help menu item in Photoshop.