I have the same issue but on Ubuntu 10.04, where Chrome works fine but FF5 displays blank boxes.

Update: I closed FF, checked for updates via the terminal, did a GET on FF again and then reopened the program and the videos now display fine. I have no idea why it worked though, since I didn't actually install anything new.

Thank you for replying! I reinstalled flash, tried a FF update (nothing new) and after a system restart it finally worked! Why? I don't know either!

How do I do that?

Same problem, Snow Leopard, FFox 5.0, updated Adobe Flash. Problem started ca. July 1. Safari works fine.

how do I check for updates via the terminal ? thanks :]

I decided to install flash on FF and uninstall the Flash ActiveX from IE. But YouTube and other flash sites were running fine on IE. I just plan on not using IE anymore, completely switching to FF. But, now that Flash plugin is installed, YouTube shows only a black rectangle where the video is supposed to be. Tried in Safe Mode with add-ons disabled, but same thing. Uninstalled the flash plugin and downloaded the latest installer from Adobe, but didn't work either. My FireFox 5.0 is also up-to-date when I checked for updates. YouTube is the only site that doesn't work. Other flash-based sites work fine! System is Windows 7 64-bit.

Please help me, I love FireFox since I started using version 3 on WinXP and it had never made me sad like this before =/

Anything I can do to help you figure out what bug is this, I'll do it. Since I'm not the only one having this very same problem! =(

How did you do the same thing? How does one check for updates via the terminal and does a "GET" on FF, running Windows 7? I have no idea and I bet many other people here don't either.

I'm having the same issue with FF5 on my mac, rockmelt and safari are working fine with youtube, ff5 displays a black box and nothing happens, till a new update is released fixing this!

Found a solution but its stupid and annoying. Can't figure out why, but if you replace http:// with https:// the video starts running. ie:

http://www.youtube.com/watch?v=k0TONBwqc7E&feature=aso <-doesnt work https://www.youtube.com/watch?v=k0TONBwqc7E&feature=aso <-works

That worked for me. Mozilla used to by my fauvorite browser, but this is really annoying. Please fix it.

The advice about using terminal and "get" commands is for folks running UNIX (Ubuntu, Linux, etc), not Windows. It might work for OS X but I don't have one handy to try.

I've got the same problem with Youtube on a Windows XP SP3 and a new Windows 7 box, both running the latest FF5 with no blocking addons and up-to-date operating systems.

Flash is installed and up to date according to http://www.mozilla.com/en-US/plugincheck/

See the attached image for an example of what's happening. It's not very exciting - just a black box with no play buttons, no error messages, etc.

The same Youtube video plays fine in IE.

Same here on my Mac. Like you, Safari works fine.

Can't figure out why, but if you replace http:// with https:// the video starts running.

This didn't work on my Mac.

using https:// works for youtube, but unfortunately it's not just youtube that I'm having problems with. It seems that pretty much any video from any site (including iPlayer) won't load in FF5, although other browsers work fine. All my add-ons and plugins are up-to date.

I had this problem when i installed FF5...i tried an update on FF (nothing new), i downloaded and reinstalled Flasfplayer 10.3...closed FF and restarted it but nothing again...at that moment i just restarted the PC and then there it is! everything worked fine!!! why? i really don't know! i mean...all it needed was a restart??? i'm sorry i can't be any more helpful! Also in "Options" i see that most video extensions are played via QuickTime Plugin 7.6.9 (i don't know if this has anything to do with this problem...i'm just mentioning it)

this seems to be fixed, it's working properly today.

Indeed it started working properly here, too! Thank you, whoever fixed it, it would be nice if you explained to us why it happened.

Are you using a Mac or PC?

I've been having the same problem for a couple of days. It was still not working this morning. I read on another thread that clearing your cache might solve the problem. I cleared mine...and now YouTube is working fine. So if all else fails you might want to give that a try too.

IT'S WORKING!!!

Earlier today, I went checked all my plugins at http://www.mozilla.com/en-US/plugincheck/, and updated shockwave, flip4mac, and silverlight. Youtube still not working.

Then this afternoon, there was a firefox update. That did it. Now, youtube works just fine.

I'm using a Mac.

I spoke too soon. Youtube back to the dreaded black box again!