It is no longer possible to use legacy extensions with Firefox 57, as many of the internal API's that those extensions depended on have been removed. If you're looking for a replacement for a legacy extension, there are several lists of suggested alternatives available:

https://mozilla.github.io/extension-finder/ https://docs.google.com/spreadsheets/d/1TFcEXMcKrwoIAECIVyBU0GPoSmRqZ7A0VBvqeKYVSww/edit#gid=0

If you can't find the addon you're looking for on one of those lists, you can also ask at the add-ons forum.

Hope that helps!

As user1929 mentioned, Firefox doesn't allow legacy extensions as of v57. You can use Nightly or Developer Edition to continue using legacy extension but keep in mind that those add-on may not work as before since APIs have been removed.