[RFC] build-x86-images: use non-ESR firefox

[paper42]

Most people probably want to use the latest non-ESR Firefox. ESR versions make sense for people who have to (or want to) stay with an older version of Firefox and still get security updates. Firefox on live isos doesn't get a security update until the package is updated, so ESR and non-ESR versions are the same in that way.

This is marked as RFC in case there is a reason why we should stay with Firefox ESR.

cc @Duncaen