By default, Mozilla Firefox blocks GPU acceleration for WebGL and certain other content in remote sessions on Windows Server operating systems. To determine what content Firefox is blocking, run Firefox in your XenApp remote session and enter the address about:support into the browser address bar. In the section labelled "Graphics" you will see the following content disabled (as of Firefox version 40.0.3):

Direct2D Enabled             Blocked for your graphics card because of unresolved driver issues.
DirectWrite Enabled         false (6.3.9600.17999)
WebGL Renderer             Blocked for your graphics card because of unresolved driver issues.

The text "Blocked for your graphics card" is misleading as, in this case, it is not the specific graphics card but general Windows Server operating system remote sessions that are blocked. Mozilla’s published advice on how to override the graphics driver blocks also applies to XenApp sessions on server operating system (see bottom of this Mozilla Wiki - Blocklisting/Blocked Graphics Drivers). In particular, entering the address about:config into the address bar and setting webgl.force-enabled=true will enable WebGL for XenApp on server operating system. Users should be aware and note the caveats and warnings that Mozilla advises if taking this route.

This issue only affects server operating system (example - Win 2012 R2 and Win 2008 R2) and XenApp. It is believed there is no issue with Firefox and XenDesktop used with Workstation operating systems (Win 7, Win 8.x etc).