changing font size pull down box not working

using firefox 56.0 and Linux slackware-current, I go to settings and then font options, but neither right or left lick will let me change the size of the fonts.  there is an arrow for a pulldown window t6o select font height, but nothing i try will activate it so I can choose the font height.

This is not the case with the others fonts on the page, just with the last font size on the font0options page.  pull down boxes on other pages of the settings work fine.

Also, the biggest the other fonts go is 14 pitch, and on 3  30" monitors, the group names are unreadable.  That was the font I set out to change.  What sets the font of the group names?

In general, 12 or 14px is not enough when your exwindow screen is 5120 x 2560 pixels.

Not everybody has that screen real-estate, but the program should accommodate those who do so they can at least read the fonts on the screen.

Thanks