Try https://addons.mozilla.org/en-US/firefox/addon/zoom-page-we/ instead.

The Web Extensions API is Mozilla's attempt to get various plugin-related
performance problems under control. While Firefox had its own performance
problems, the users who complained that Firefox would drain their laptop's
battery in 3.14 seconds often had some extension that "helped". So Mozilla
made a whitelist of functions plugins can call, took advantage of the
compatibility break to make some other improvements, and announced that
starting with FF57 plugins can only use the whitelisted functions.

It seems possible to modify Zoom Page to the new regime. Anyway there's a
working alternative.

Arnt