For very long email with few pages of screen, currently attachments are placed at the end, you have to scroll few screens to reach them, so it is better to place them at top, or a way to jump to the end.
- skip 'Enable userChrome customization in about:config'; you've already done that
- follow the instructions under 'Locate and open your profile folder..' NOTE: for TB, the menu choice is Help > Troubleshooting Information.
- follow the instructions under 'Create the folder and its files' (make sure userChrome.css doesn't end with .txt - Windows must be set not to hide extensions for known file types)
- once userChrome.css is created, open it and enter the code I mentioned above
- save the file > ignore the rest of the instructions > restart TB