Since I updated to Firefox 70.0.1, I see this spammed (meaning it's so present it make it hard to find something in there) in the browser console:
I think this may happen whenever a tab is closed, and possibly in other situations.[Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMWindowUtils.removeSheet]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: resource://gre/modules/ExtensionCommon.jsm :: runSafeSyncWithoutClone :: line 75" data: no] ExtensionCommon.jsm:75:12
runSafeSyncWithoutClone resource://gre/modules/ExtensionCommon.jsm:75
cleanup resource://gre/modules/ExtensionContent.jsm:413
I confirmed with the "disable all addons, then toggle them 1 by 1" method, that the source of the issue is indeed Adblock Plus (version 3.7).
I hope it will get fixed.