Are you using the developer version or the stable version?

Interesting information about latest problems with userscript greasemonkey/greasemonkey#1904
I rollback nightly greasemonkey build to stable 1.15 and latest dev script work fine.
Apparently all the recent problems with the script because of this.

@YePpHa I use the addon for firefox not the userscript. I'm not sure if it's a dev version but I didn't change anything so it's probably release.

@RX14 Might I recommend you to try installing the developer version instead to test if it has been fixed. I've done a lot of bug fixes for the Firefox addon since the release (also I minimized a memory leak).

For my own usage in the very near term I "fixed" this by editing youtube center to 1) @require-ing the grant none shim and 2) explicitly set @grant none, and things at least don't catastrophically fail like this.

This will break cross-domain XHR, but I don't even know what youtube center wants that for. Does it actually need to generate cross-domain requests? I'm not sure what other features this might break.

If you set @grant none YouTube Center will not have access to the variable unsafeWindow, which I guess is the culprit in this issue. Anyway if the unsafeWindow is not available YouTube Center will inject itself into the page and therefore not needing the unsafeWindow, but that means it will not have access to storing the settings by using Greasemonkey's API or access to the cross-domain XHR.

YouTube Center uses cross-domain XHR to check for updates, loading the YouTube Experiments list and to load a JavaScript page, which it will decode to decipher a scrambled string of characters, which is needed to download some YouTube videos. Other than those few features YouTube Center will still work.

I took a closer look and the shim is indeed not necessary, the script implements equivalents itself, for cross-platform functionality it seems.

So just @grant none makes it not fail catastrophically, like the original report describes. But breaks features, as called out above, which rely on those privileged APIs which are no longer available.

edited:
My mistake, while firefox extension builds doesn't cause black screen like userscript they don't work correctly either.

Errors in console: http://pastebin.com/pGfc3cmP
debug: http://pastebin.com/Y5QcPJEx

I'm having same problems with Aurora build, but only with 720p HTML5 videos, 360p videos show up correctly, and I'm using the extension version.

with greasemonkey version youtubecenter looses settings and with extension videos some are not playing and it looks like lots of settings do not work, here is my log http://pastebin.com/dYxMF71g

developer version #270 seems to work fine

Can someone else confirm that developer version 270 works?

I can confirm # 275 works.

none up to #279 work 100%

When using XPI there is no black screen, but embeded players does not work.

When using .user.js black screen, both in embeded player and on YT page.

Some rrrors in Browser Console for both XPI and .user.js

@kulmegil could you please post these errors you see in the console? They might help me in understanding what's causing the issue.