As I wrote in the Firebug discussion group, that the <body> element is selected first before the actual element you inspected is a bug and I've already filed a bug report for it.
There is also a bug related to the Inspect Element with Firebug option, which opens the DevTools, but doesn't actually select the inspected element. This bug got fixed in Firebug 2.0.19 by removing the option (so there's just the Inspect Element option left, which works as explained above).
As mentioned in my answer to a related thread, you may be able to reenable Firebug by setting the preferences browser.tabs.remote.autostart, browser.tabs.remote.autostart.1 and browser.tabs.remote.autostart.2 to false via about:config.
These settings control whether Firefox works in multi-process mode (separate processes for its UI and the websites). Unfortunately Firebug is incompatible with that mode.
Note: This is a temporary solution, because the multi-process mode will be mandatory at some point and the preference will be removed. Also, starting from Firefox 57 only WebExtensions will be supported and Firebug is based on different APIs, so it definitely won't work anymore at that point.
Another way is to install an old version of Firefox like 48, disable automatic updates and install Firebug there. Though, having said that, this option is not recommendable (at least not long term), because you will miss important (security) bug fixes and new features.
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