[john70t]

Update Firefox.
Install NoScript as an addon.
Disable all other addons for now.

Then enable them all slowly, one for each restart, until the problem comes back.
Boom. There is the suspect addon!!! Remove it permanently.
The situation should be obvious if the contamination is only in the browser.

You may need to uninstall the whole browser with CCleaner (which also has a registry cleanup) and then re-install a fresh version direct from the Mozilla website.

On a deeper O.S. level:
Run Malwarebytes and/or MSSE (Microsoft security essentials).

It's worth double-checking the processes running within the Operating System. (CNTRL+ALT+DEL).
Do an internet search for "process library". Most sites for this are ambiguous and difficult to understand, but it's worth getting to know the usual happenings.

(edit) oops. I didn't read this is a mac machine. YMMV.