Firefox will work in most scenarios. However, at some points, SharePoint is still using ActiveX components, so Firefox won't work there.
For instance: the Datasheet View option for a list in the ribbon will be disabled.
Also, Explorer View for a document library will be disabled.
In short: most limitations come from the use of ActiveX components, the reason why you get the same limitations for Internet Explorer 64-bit, 'cause the ActiveX components are 32-bits...
I also ran into problems with opening Excel documents: