I am in Firefox also. Are you on a desktop computer, laptop, tablet or phone? The main desktop version of AirForums or the App? The following comments are for a MacBook Pro laptop, and the main desktop version of AF -- FWIW.
When you do the first draft of the post, there is a choice at the top left of the text composition box [where I am typing right now] -- for the font and text size.
If you are editing a post, you have to click on the "Go Advanced" button below this text box, where you can choose font/size.
I think you can make your font and size preferences into permanent "defaults" in the User CP at the top left, in the blue menu bar above:
User CP>Options>Misc. Options>[stumble your way through -- too many options to sum up here IMO].
Also the FAQ link in the same menu bar above has:
FAQ>Board FAQ>Reading and Posting Messages>[a variety of choices which may include font/size IMO]
Also the "Forum Admin" forum may contain more helpful comments, than this "Airstream Lifestyle" forum does IMO: