One of them thar new-fangled computer thingys running Windows and Firefox v60.0.1 (64-bit)...
I've seen that, too, when using my Windows 10 with Firefox. Just random posts will have Edit missing (not the same as when it's missing from all posts and a sign out/in fixes). For at least one that I really needed to edit, I just opened a new browser. So, it may be a weird Firefox glitch.
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I'm being extremely clever up here and there's no one standing around looking impressed. What's the point in having you all? - Dr Who