I see you've already done a broad search but first thing I'd try is searching 738dce45-51d7-46d1-aa04-f49e33ce11af in your user.js file or even in about:config. Your search method may not have checked within preference/reference files, I have no idea though. It may just be leftover pref. data from an older or uninstalled extension.

Other thought is it may be just the older background for an extension. Were any of them updated recently?

If you open the Add-ons Manager and look at the More... link for each installed extension, all the way at the bottom there is a Last Updated date that should allow to to figure out the most recently updated (or installed) extensions, which nay help you pinpoint the culprit.

Something was changed in Firefox in the last version or two that is triggering those moz-extension://... lines to appear in History, but I haven't been following those forum threads.

it occurs in extensions.json and prefs.js in the profile folder. apparently it belongs to AwesomeScreenshot. here's the relevant part:

"optionsURL":"moz-extension://738dce45-51d7-46d1-aa04-f49e33ce11af/options.html","optionsType":5,"aboutURL":null,"icons":{"16":"images/icon16.png","32":"images/icon32.png","48":"images/icon48.png","128":"images/icon128.png"},"iconURL":null,"icon64URL":null,"defaultLocale":{"name":"Awesome Screenshot - Capture, Annotate & More","description":"Screen capture for all or part of any web page. Add annotations, comments, blur sensitive info, and share with one-click uploads.","developers":null,"translators":null,"contributors":null}

But that extension was updated in Sept. oh well.

See also: What is moz-extension and what purpose does it serve (Firefox v48.0)? - Awesome Screenshot Plus