My problem is with OS X 10.10.2 and FireFox 35.0.1. Same thing on my iPad.The dialog box comes up but the select files button doesn't do anything. I just click on basic uploader and it works going that way.
The regular uploader uses flash, and may have issues with some devices and browsers. vBulletin had removed the flash uploader for some time as there was a security vulnerability in the YUI source for the .swf uploader. It has since been reactivated.
I can disable it all together, and force everyone to use the basic uploader, but since the flash uploader works fine in Chrome on Windows and other browsers, then why disable something that works for the majority?
Unfortunately for you with OS X Yosemite, it doesn't work with FF, Chrome, or Safari, and you'll have to use the basic uploader for everything.