Does that work if you’ve posted from your phone, though?
That may be a dumb question
Not a dumb question at all!
Missing E will only work when you post through either Firefox, Google Chrome, or Safari where you have the extension installed.
For example, if you install Missing E on Safari on your laptop, and then post to Tumblr using Google Chrome on your laptop, the settings will not be carried over.
(You can export your settings from one browser and import them on another.)
There is no Missing E for your iPhone, iPad, etc.