Dear Dan
for forum and web site tests I always use 3 different browsers: IE, Chrome, Firefox. For all I always have latest version. My preferred is actually Chrome.
I also use different computers for making tests, my home one (I have 3) and from office.
Here all seems working as expected in all browsers otherwise I would have act immediately and made a post in case I was not be able to fix the problem.
Please check your Firefox settings. The fact you see so big font maybe is related to the zoom factor (now all browser can zoom in/out any web page)
Let me know.
Eros