If you have an issue please first check if someone else have the same issue as you. If not then create a new issue at the Issue Tracker.
This is caused by YouTube limiting the availability of qualities when DASH Playback is disabled. DASH playback is available for the flash player, but is more limited for the HTML5 player. The HTML5 player needs MSE (Media Source Extensions) to be available otherwise DASH playback would be disabled. MSE is disabled in Firefox, but can be enabled in Firefox 31 (see this guide on how to enable MSE).
A new version of YouTube Center will get released when I think that it's ready. If you're impatient you can try the developer version.
If you don't care about the download feature in YouTube Center I would recommend you to install the Chrome WebStore version instead, which can be found here. You can also use Tampermonkey and install the userscript version.
If none of the options above works for you, you can also unpack the extension and load it through the developer mode.
.crxfile by using 7-zip or some other tool. Please note that you will have to choose a place where the extension would be saved at as it will be used by Chrome.
chrome://extensionsin your Chrome browser and make sure that the
Developer modeis enabled.
Load unpacked extension...and select the folder you unpacked ealier.
.crxfile and unpack it at the same location, where you then just have to click on
Reload (Ctrl+R)link in the
chrome://extensionspage to make sure that Chrome loads the updated version.