The easy approach is to save a copy of Firefox 54 and disable its update features. This means that you will keep the features of automating an (older version of) Firefox.

There also are WWW::Mechanize::PhantomJS and WWW::Mechanize::Chrome which currently are able to automate more "current" browsers, but PhantomJS is also nearing its end of life.

All three modules try to maintain a similar API so you should maybe be able to switch to the other modules.