Hi Melisa,
the grid overlay is a feature of the Firefox built-in DevTools and is not part of Firebug, which is
discontinued.
What you describe sounds like a bug to me, though unfortunately I cannot reproduce it. I suggest you first check whether you have the latest Firefox installed (54.0.1, tomorrow it will be 55.0). If you have the latest version and still see the issue, try to reproduce it on a fresh
Firefox profile. If it works on the fresh profile, your normal profile must be somehow corrupt. This might be a conflict with an add-on or a preference. So, you should then try to disable all the add-ons and check whether it's working. If so, enable each add-on again one by one to find the culprit. If it's not related to the add-ons, it's probably a preference. In that case you should compare the preferences of your normal profile with the one of the new profile.
If the new profile also has this issue, first check if it still exists in
Firefox Nightly. If the answer is yes, then you should
file a bug and if possible attach a test case or URL allowing the Mozilla people to reproduce the problem.
Best regards,
Sebastian