Yep, you're all good. I'm surprised it let you remove the fix from about:addons but I wouldn't worry about it.
That value resetting is expected, it's the time when Firefox thinks it last checked signatures, it resetting just means this convinced it to recheck as intended.
As a side note, the value for that setting got reset after I restarted firefox.
Also, I uninstalled the fix by simply removing the extension in about:addons. Is this the correct way to do it?