Might be wrong on this, but initial guess is that we don’t define browser list and it’s using some default (probably @babel/preset-env · Babel) and doesn’t consider this version as something we support and as a result is not transpiled.
It’s weird though firefox 74 is only one year old, but yes looking at browserlist currently:
$ npx browserslist
...
firefox 89
firefox 88
firefox 78
...
Now, the question is, @sam do we want to support this? Altough it’s only one year old, current fx version is 89, so this is 15 versions behind. Not sure about this your call. It’s important to consider that to support this it will add weight to everyone’s payload as transpilling basically works more or less like a polyfill (adding some functions tu support this and replace the syntax by functions calls).