This Firefox add-on helps you upload better versions of existing posts to e621.

Installation

1) You need Firefox 42.0 or newer

2) Download the add-on:
https://drive.google.com/file/d/0Bx_JbLEooF94U0d6YzlaQTFIMWs/view?usp=sharing

3) Open the .xpi file in Firefox and click "Install Now"
=> Firefox icon with the text "ubv" appears in the Firefox toolbar (upper right corner)

Usage

1) Open three tabs:
- inferior e621 post, like https://e621.net/post/show/387377
- gallery page with better version, like http://www.furaffinity.net/view/11789186/
- direct image link, like http://d.facdn.net/art/dirty-maxi/1381282367/1381282367.dirty-maxi_tittylickingood.png

2) Left-click on the UBV icon
=> pop-up opens

3) Select the tabs:
- Ctrl-Tab to the inferior e621 post, and click "(De)Select e621 tab"
- Ctrl-Tab to the gallery page with better version, and click "(De)Select artsite tab"
- Ctrl-Tab to the direct image link, and click "(De)Select direct image link"

4) Click the "Upload" button

If there's an error, the add-on displays it in the white text box at the bottom of the pop-up window.
- temporary error (too many people using e621 at the moment) => click the "Resume" button to try again
- permanent error (the version you're trying to upload is already on e621) => click the "Clear" button to empty all fields

Configuration

When the add-on has uploaded the better version, it flags the inferior one for deletion.
If you're a Janitor or above, you can configure the add-on to delete the inferior version immediately:
1) Open the Add-ons Manager
2) Click the button "Options" next to "Upload Better Version"
3) Select the radio button "Delete the inferior version (Janitor+ only)

The add-on posts the message "Better version of post #xyz" on the superior version.
You can turn it off in the configuration.