Rated 5 out of 5 stars
As much as I like the new Firefox, I will miss this addon.
This is not a bug that can be fixed. Firefox 57 is the first version of Firefox that only allows modern WebExtension addons, so iMacros as powerful XUL-based addon no longer runs.
As alternative I switched to the Kanto extension, but it is not available for Firefox yet, only Chrome.