My Font has reduced drastically

Hi, while I was typing on this site earlier today my font size dropped to a very small size, I am using Firefox, I'm not sure what happened but I can hardly read what I am typing. ( I am slightly sight challenged) Could someone please direct me to where I can change this as I have searched for an edit button with no avail. It's probably too small I can't see it LOL.

It is only relevant on this site as when I go into other sites all is well.

Thanks in advance

Cheers

D