In case you want to later correlate such an URL to the actual extension:
Go to about:debugging in the browser, select “This Firefox” (or other release channel name). You should then see a list of all extensions (possibly after enabling debugging, if it prompts you for it), with three entries per extension, the first one is the ID, the second one this profile specific UUID and the third one will be a moz-extension URL to the extension’s manifest. Using the last two, you should be able to find the extension that matches the error URI (you can use the find in page feature for convenience).
If this doesn’t uniquely identify which extension it is, or you just want to be very sure, you can find the extension by going to https://addons.mozilla.org/addon/<Extension ID> where you replace <Extension ID> by the ID shown in the first of the three lines. If this gives you an error page, this is likely an unlisted extension.