Fixes #40137 (closed).
One of the problems was ExtensionStorageIDB.setMigratedExtensionPref being called when the "new" https-everywhere-eff was installed, which prevented the migration. The patch makes the https-everywhere-eff extension install/uninstall behave as if the prefs extensions.webextensions.keepStorageOnUninstall and extensions.webextensions.keepUuidOnUninstall were set to true, which should allow the migration to be executed correctly when we switch to 10.5.