Is this about a font for the Firefox user interface or the default font used on web pages?

Make sure you allow pages to choose their own fonts.

• Options/Preferences -> General: Fonts & Colors -> Advanced
  [X] "Allow pages to choose their own fonts, instead of my selections above"

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

Did you do a font check in the FontBook is case some fonts are corrupted?

I did not understand any of the references given by respondents--who are probably tech-smart--and not people like me who have such basic problems as setting a font size. I need uncomplicated, simple answers--and get lost when reference is made to features or functions I know nothing about--such as below:

"Attach a screenshot" you wrote. Huh? What's a screenshot? (a photo of the screen?) and how do you attach it? (notice I'm being led away from my original question about changing font size)

"use a compressed image type like PNG or JPG to save the screenshot"--yikes--I'm asking how to make a font size larger and the respondent uses terminology and refers to features that are a whole new vocabulary with which I'm unfamiliar.

So--the screen has 30 emails--all with tiny font size (previously a good size but I must have hit something that diminished the font)--so how do I return it to 12 pt., for example?

Hi mfs1, Just FYI - a screenshot is similar to the image below - which is what @cor-el was referring to above.

To get to your issue - Without knowing what web-mail you use, I can't tell you where to go in the settings to reset the default font size. It should be in the settings, though.

In Gmail for example, you can easily change the font size with Ctrl - - (minus key) to decrease the font size and Ctrl - + to increase the font size. Pressing Ctrl - \ will reset the font size to default.

Thanks much for replying so swiftly. Right after posting what I did, I went to every icon and clicked on each one--to see what each gave me. Surprisingly, I FOUND what you reported today. Now I know something new :):)

I represent the masses "out there" who really have a problem navigating computer programs--even though I'm at the computer for hours a day with Word and emails.

So, thanks for reaching out to us who are challenged.