With the help from comment 20, I've succeeded in "activating" Composition = WebRender (Software) on every one of the Windows systems I have, I could still not reproduce any kind of crash or failure. Unfortunately, this issue appears to be intermittent; a fact also supported by comment 12. Could not be properly verified unless reproduced. In any case, no new (and relevant) regressions were discovered in builds after the fix.
Bug 1722054 Comment 23 Edit History
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With the help from comment 20, I've succeeded in "activating" Composition = WebRender (Software) on every one of the Windows systems I have, but I could still not reproduce any kind of crash or failure. Unfortunately, this issue appears to be intermittent; a fact also supported by comment 12. Could not be properly verified unless reproduced. In any case, no new (and relevant) regressions were discovered in builds after the fix.