If you download the installer again from

http://www.mozilla.org/firefox/fx/#desktop

What happens when you try to run it?

On YouTube, are you saying that the videos do not start automatically, or they do not start when you click on them?

If you have not updated your RealPlayer add-ons in the past month or two, you might want to update those.

A standard diagnostic for interference by add-ons is to try Firefox's Safe Mode.

First, I recommend backing up your Firefox settings in case something goes wrong. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Firefox's Safe Mode (Diagnose Firefox issues using Troubleshoot Mode) using

Help > Restart with Add-ons Disabled

In the Safe Mode dialog, do not check any boxes, just click "Continue in Safe Mode."

If YouTube works correctly in Safe Mode, this points to one of your extensions or perhaps a custom setting as the problem.

Any change?

You can check for problems caused by recent Flash updates and try these:

• disable a possible RealPlayer Browser Record Plugin extension for Firefox and update the RealPlayer if installed
• disable protected mode in Flash 11.3 and later
• disable hardware acceleration in the Flash plugin

• http://kb.mozillazine.org/Flash#Flash_Player_11.3_Protected_Mode_-_Windows

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"