You should be able to try it if you enable webgl.enable-privileged-extensions, though this is *unsafe for the general web*. Note that Firefox does not have a different EXT_disjoint_timer_query_webgl2, and uses EXT_disjoint_timer_query for both webgl1 and webgl2.
Bug 1328882 Comment 5 Edit History
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You should be able to try it if you enable webgl.enable-privileged-extensions, though this is **unsafe for the general web**. Note that Firefox does not have a different EXT_disjoint_timer_query_webgl2, and uses EXT_disjoint_timer_query for both webgl1 and webgl2.