What happens if you disable Flash here:

orange Firefox button (or Tools menu) > Add-ons > Plugins category

Can you visit Youtube without problems or are there still issues?

Obviously large parts of the web fail if Flash is disabled, but for Youtube, you could try the HTML5 trial: http://www.youtube.com/html5/. Of course, that could cause its own problems...

Thank you for your reply! Well, it turns out the problem isn't with the videos or Flash. The problem is with Youtube itself. I discovered this when I decided to look for embedded videos. Lo and behold! I have no trouble playing Youtube videos that are embedded in other pages. The freezing happens only when I visit Youtube itself - the video pages on the actual site are what's freezing FF. So it seems it's not a Flash problem at all. (Also, this problem does not happen on any other video site, only Youtube, and only with FF.)

So I'm really puzzled. This has never happened before, with any site. Just out of the blue, FF has decided it hates Youtube.

Do you have any add-ons related to video, such as downloaders? If so you might try disabling them to see whether that makes any difference. You can do that here:

orange Firefox button (or Tools menu) > Add-ons

Check both the Plugins and Extensions categories.

Youtube, like other Google sites, is a complex and frequently changing application. When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

Then try reloading the page. Does that help?

Also, do you usually use YouTube logged in? I generally do not log in, so I'm not sure whether I'm replicating your setup.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thanks to everyone for your suggestions!

I have no idea what happened or why, but the problem suddenly disappeared yesterday. Youtube works fine now. It's a complete mystery to me, seeing as how I didn't actually change anything.

But I appreciate all your help, and will keep your answers in mind should it ever happen again!

I just removed Video DownloadHelper 4.9.15 add-on and voila, Firefox doesn't freeze anymore. If you have it you better remove it. It worked for me maybe it will work for you!

I think you know how to remove an add-on but if you don't, read below:

• In order to uninstall a possibly unwanted extension, please do the following:

1. From the Firefox window click the Firefox button at the top left and select Add-ons, or, if the Firefox button is not shown, click the Tools menu and click Add-ons.
2. Once the Add-on Manager has opened in a new tab, click the Extensions button on the left side of the window.
3. You should now see a list of your installed extensions on the right side together with buttons on the right side of each extension.
4. To remove an extension from Firefox, simply click the Remove button. You should see a message that informs you about the successful removal of the add-on.
5. Note that some add-ons require a Firefox restart to be removed completely. To perform a Firefox restart after the add-on removal, click the Restart now link in the message.

• You can find further information about uninstalling extensions in the following articles: Disable or remove Add-ons

Did not work. Here is what happens. I am watching a U-Tube video. I leave the video before it is done, or stop the video. Only when I do this does it seem that Firefox stops working. But maybe it is the ability to download anything from the Internet that stops. Firefox sends out the URL page request --but never get a response, and nothing further (web pages) download. Other software also can send packets but nothing ever comes back. Maybe the firewall? It is AVG Internet Security 2013. Only solution has been to re-boot the computer. I had this problem with this computer when it was an Windows XP operating system. This computer has been upgraded to Windows 7. But still having the same problem.