I just installed OSX Mountain Lion, and now Safari doesn't work...

Turn off Extensions in Safari > Preferences > Extensions. Quit and relaunch Safari to test.

If it's not an extensions issue, back to Safari > Preferences. Select the Security tab.

Deselect: Enable plug ins

Quit and relaunch Safari to test.

If that helped, there are one or more Safari unsupported add ons installed that were compatible with Lion and a prior version of Safari but not Mountain Lion.

Troubleshooting help here.

And if it applies, try temporarily disabling anti virus software.

Please read this whole message before doing anything.

This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.

Step 1

The purpose of this step is to determine whether the problem is localized to your user account.

Enable guest logins* and log in as Guest. For instructions, launch the System Preferences application, select Help from the menu bar, and enter “Set up guest users” (without the quotes) in the search box. Don't use the Safari-only “Guest User” login created by “Find My Mac.”

While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.

Test while logged in as Guest. Same problem(s)?

After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.

*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.

Step 2

The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode* and log in to the account with the problem. The instructions provided by Apple are as follows:

1. Shut down your computer, wait 30 seconds, and then hold down the shift key while pressing the power button.
2. When you see the gray Apple logo, release the shift key.
3. If you are prompted to log in, type your password, and then hold down the shift key again as you click Log in.

Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs.

The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

*Note: If FileVault is enabled, or if a firmware password is set, or if the boot volume is a software RAID, you can’t boot in safe mode.

Test while in safe mode. Same problem(s)?

After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of steps 1 and 2.

Have you emptied the Safari cache and history list?

Read this whole message before doing anything.

Back up all data if you haven’t already done so. Before proceeding, you must be sure you can restore your system to the state it’s in now.

Quit Safari if it’s running. Then select  ▹ Force Quit... from the menu bar. A small window will open with a list of running applications. Safari may appear in that list, even though you quit it. If so, select it and press return. Close the window.

Step 1

In the Finder, press the key combination shift-command-A to open the Applications folder. Select the Safari icon in that folder and press the key combination command-I to open the Info window. There’s a checkbox in the Info window labeled Open in 32-bit mode. Uncheck it, if checked. Close the Info window and the Applications folder.

Hold down the option key and select Go ▹ Library from the Finder menu bar. Delete the following items from the Library folder (some may not exist):

Caches/com.apple.Safari

Caches/com.apple.WebKit.PluginProcess

Caches/Metadata/Safari

Preferences/com.apple.WebKit.PluginHost.plist

Preferences/com.apple.WebKit.PluginProcess.plist

Saved Application State/com.apple.Safari.savedState

Leave the Library folder open. Try Safari again. If it works now, stop here. Close the Library folder.

Step 2

If you still have problems, quit Safari again.

Launch the Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Terminal in the page that opens.

Drag or copy — do not type — the following line into the Terminal window, then press return:

rm -fr $TMPDIR../C/com.apple.Safari

Quit Terminal. Launch Safari and test.

Step 3

If Safari still doesn’t work right, quit, go back to the Finder and move the following items from the open Library folder to the Desktop (some may not exist):

Cookies/Cookies.binarycookies

Preferences/com.apple.Safari.LSSharedFileList.plist

Preferences/com.apple.Safari.plist

Preferences/com.apple.Safari.RSS.plist

Preferences/com.apple.WebFoundation.plist

PubSub/Database

Safari

(Note: you are not moving the Safari application. You’re moving a folder named “Safari.”)

Try again. This time Safari should perform normally, but your settings and bookmarks will be lost.

If the issue is still not resolved, quit Safari again and put all the items you moved to the Desktop back where they were, overwriting the newer ones that will have been created in their place. You don’t need to replace the files you deleted in step 1. Stop here and post again.

If Safari is now working normally (apart from the lost settings), look inside the “Safari” folder on the Desktop for a file named “Bookmarks.plist”. Select File ▹ Import Bookmarks from the Safari menu bar. Import from that file. Recreate the rest of your Safari settings. You can then delete the items you moved to the Desktop.

Note: This step will remove your Safari Extensions, if any, and their settings. If you choose to restore them, do so one at a time, testing after each step to make sure you haven’t restored the problem.

If you don’t like the results of step 3, you can undo it completely by quitting Safari and restoring the items you moved or deleted in that step from your backup, overwriting any that were created in their place.

It removes a hidden and seldom-used cache folder.

Please read this whole message before doing anything.

This procedure is a diagnostic test. It won’t solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

Repeat with this line:

You can then quit Terminal.

With reference to my post timestamped "Oct 1, 2012 8:50 AM," did you stop at step 2? If you did, please go to step 3.