Sorry for all the typos. I was on my way out the door and didn't have time to try different browsers.
I suggested the Safari Develop settings, because you didn't mention that Firefox was the browser experiencing/causing the problem.
Based on a couple of simple tests below, clearing the cache and saved cookies for all the problem websites would be the first thing I would try.
Do you have Firefox automatically check for updates to browser and add-ins?
I had an older Mozilla Firefox release (10.x) that kept crashing when I started it, before even before loading add-ons or navigating to any website. Downloading and installing the latest version of Firefox 15.x directly from Mozilla fixed that problem. The polariscatalog.piercecountylibrary.org website link opened in Firefox 15 just fine, and completed a simple search without errors.
I usually use the Mozilla Aurora browser beta rather than Firefox. It also opened the full website automatically and a simple search and retrieval worked.
Google Chrome worked, opening the full site.
So none of my Mac browsers attempted to automatically open mobile version.
I also tried opening the site on my wife's iPad 2 running iOS 5.1.1, to see if it detected it as a mobile device. The full site opened without errors in iOS Safari.
Using mobile site link in the upper left corner of the full library website, opened a simple text screen without graphics, and showed the same behavior as clicking the mobile site link in all the OS X browsers and iOS.
Message was edited by: kostby