The ‘Display Style Information’ feature had to be removed as Firefox removed the APIs that it was using. The best replacement for this feature is the CSS inspector built into the browser developer tools:

https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Examine_and_edit_CSS

Older versions of the extension are available here, but note that they are not supported and will not work in future versions of Firefox:

https://addons.vaultpit.io/en-US/firefox/addon/web-developer/versions/