Could you try reattaching the screen shots? You can reply to your thread here: https://support.mozilla.org/questions/1054776

JD-Vincent said

I have a brand new computer and installed Nightly. On many web pages the font is tiny, thin, and unreadable -- even if I zoom in, the base font is terrible. It looks like the thinnest weight of Helvetica. On my old computer, in traditional Firefox, I had a similar issue on some pages -- the text became unreadable while I was entering a new comment on Facebook. Now, this is an issue in regular web pages. I do not think this is "zoom" issue, but rather a problem with the browser selecting an inappropriate typeface. Below, please find two screen captures, one at normal resolution, one Zoomed in (Ctrl + five times)

Hmm, yes, that's hideous.

I'm not sure exactly what page you were on, so I looked at:

http://www.java.com/en/download/installed.jsp

The page has this ranking for preferred fonts for most of the document body:

font-family: "Helvetica Neue",Helvetica,Arial,sans-serif;

On my Windows 7 system, Firefox 36.0.4 uses Arial; I apparently do not have the other two fonts.

If you right-click some text in the body of that page and Inspect Element (Q), then on the right side click Fonts, what font is your Firefox using? I am attaching a screen shot for comparison.

Edit: Added screen shot showing where you can find the effective font-family rule for the particular element.

It is supposedly using, HelveticaNeue-MediumCondObl, which is a pretty heft weight of that font. I suspect somewhere in the CSS, there's a font-weight setting turned low. (I think Helvetica Neue is font Apple uses, FYI)

That solves the mystery for this instance, but I still see small unreadable fonts elsewhere -- most commonly when adding a comment to Facebook. Short of removing Helvetica Neue from my system -- is there any way to keep Firefox from accessing it?

Actually, I just looked at the Rules tab and there's an entry for font-size:75%.

I have read about a technique for pointing font name "Font1" to the physical font file for "Font2" as a way of substituting fonts. This blog post describes the general principle and one way to create and apply such a custom style rule:

Helvetica on Firefox: Overriding Fonts with CSS | Keyvan Minoukadeh

I haven't tried it myself.