Work in Safari and Google Chrome.
Specifically, which buttons? All the links and triggers appear to work fine for me in Safari, Chrome and Firefox. What platform and what version of Firefox are you working with? Have you cleared the browser cache in Firefox?
Ah, success. I'm able to reproduce the problem with the slideshow thumbnails in Firefox. Very strange. Obviously it's a Firefox-specific idiosyncracy. I expect it has to do with the specific formatting on the different states of the thumbnails.
Could you send us your .muse file at firstname.lastname@example.org so we can investigate further. We'll probably be able to provide a workaround while we work on a fix, once we understand the cause better.
Please send the file along with a link to this thread. If the file is larger than 20Mb you can use a service like Adobe SendNow, SendThisFile or WeTransfer. We don't need the image files, just the .muse file. Thanks.