Install the official extension Pale Moon Commander, it adds an Advanced Preferences option in the tools menu. From the resulting dialog, you can configure WebGL settings like so:
The settings you see here are at their default values, so WebGL is enabled by default. I find this far better than mucking around in about:config; at least for the settings that are exposed in this UI. If you don't touch anything, you can see at a glance what each setting is at its default.
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