Is this possible? How would the OpenGL context be created in something like an ActiveX control or a Firefox plug-in? Is there any particular reason why it would not be possible to build a browser plug-in that renders using OpenGL?
That looks very interesting except that Microsoft aren’t on board (no surprises there!) so no IE support.
Are there any reasons why OpenGL couldn’t be used in my own plug-in?
Well opengl can be used in your own plugin. Quake Live does this AFAIK.
These pages can provide more help :
https://developer.mozilla.org/en/Gecko_Plugin_API_Reference/Plug-in_Development_Overview
Maybe it is easier to create a windowless plugin, that create a GL window like a classic GL program.
Off-topic but still interesting, speaking of WebGL these a neat tool to play with GLSL shaders in your browser :
http://www.iquilezles.org/apps/shadertoy/