There are also options such as:
User Agent Switcher
(Also available, query for it, OP asked about Firefox, so priority)
This does NOT however change your native screen resolution, which is also typically passed.
So, for instance, if you are trying to tell a site that you "are" a Windows phone you can, but know that Google adwords, analytics, and anyone else who cares, leverage screen size resolution to aid in fingerprinting/marketing so it is nothing new.
Macbook Pro screens only come in a few sizes.
Check out what you are passing here: