Working fine here on Firefox 57.0.2 (the latest stable release):
What version are they running? (visible at about:support)
Are there any errors in the console? (visible with Ctrl Shift k, or Menu → Web Developer → Web Console)
Does it work when all add-ons are disabled? (you can restart Firefox in safe mode by going to about:support and hitting Restart with Add-ons Disabled...)
Does it happen on all Discourse instances, or just yours?