Safari has problems displaying WAY TOO MANY pages.

Question

For some time now, I've noticed that Safari simply cannot display certain pages. The first time I noticed this was on my MBP17 with a Snow Leopard (10.6.8) installation, using Safari 5.1.9. After a barage of Security Updates started creating a flurry of problems on this computer (including the Safari problem, as best I recall), I finally gave in and migrated (I can't really say upgraded) to Mountain Lion. It was my hope that this would cure the recent problems. It did not! After migrating to Mountain Lion (10.8.5) and Safari 6.0.5, I still see the same thing that I get on my MBP15 that remains on OS 10.6.8 and Safari 5.1.9.
The problem I see is that Safari simply can't display certain pages. Many of them are on Apple's own website. For a while, I just thought Apple was creating pages with no content. It wasn't until I got to a page that had menus that wouldn't work (at all), that I decided to try the same page with Firefox. To my surprise, the page displayed perfectly on Firefox. I pasted the source code from pages displayed in both the Safari and Firefox into separate BBEdit documents and compared the source. It was absolutely identical! Presumably, therefore, the problem occurs with JavaScript modifications to the DOM. While it's easy to cut n' paste the DOM from Firefox, I've yet to find an easy way to do this with Safari's debugger. So, I can't tell exactly where the problem starts. Besides... Apple just doesn't pay me enough to debug their code for them!
The follow screenshots show the results of the attempt to display http://www.apple.com/support/itunes/ in both browsers. (I was on this page trying to figure out a solution to the error message "an unknown error occurred (-42408)" that I keep getting in iTunes... a whole different can of worms!)
As displayed in Safari (or as NOT displayed in Safari)...
As displayed in Firefox...
This problem crops up on many sites and is rather unpredictable. The thing that amazes me most is how often it occurs on Apple's own site! And since I see it on two different computers, with different OS and Safari installations, I can't possibly be the only one who sees this. I did just try it on my iPad 1 and the same page displays fine. Do I just need to go back to an even older OS version on my MBPs? Maybe OS 8?
Anybody have an idea what might be going on? I have spent the entire past week debugging iOS7, iTunes, Safari and Mtn Lion. After over 34 years on Apple products, I'm just about ready to go buy a PC!! Maybe even a CPM box.
PLEASE HELP!
PS: Just to make it WORSE... I have tried to enter this several times and keep getting an error. I finally had to switch to Firefox and try again. What the HEXX has happened to Apple???

Answer 1

From the Safari menu bar, select
Safari ▹ Preferences ▹ Extensions
Turn all extensions OFF and test. If the problem is resolved, turn extensions back ON and then disable them one or a few at a time until you find the culprit.
If you wish, you may be able to salvage the errant extension by uninstalling and reinstalling it. Its settings will revert to their defaults. If the extension still causes a problem, remove it permanently or refer to its developer for support.