Does it work if you manually edit this font-family? Does it show where this font comes from if you expand the message?

What does it show on the Fonts tab? You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Fonts tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/edit_fonts/

I don't know whether it's possible to zoom out so far that the text looks like a few pixels, but your screenshot is giving me that vibe. Is there a zoom level button in the address bar to reset to normal?

jscher2000 , you are spot on , seems the current font size is "1" . Now where could I edit the font size to make it bigger permanent. seems like there is Chrome CSS file that needs editing. chrome//activity-stream/content/css/activity-stream.css:739

the small font is also effecting other browser elements , like editing bookmarks , also not legible.

it turns out that the minimum font size was set to None in Advanced . Strange that this setting just altered on its own, I've never set any configurations previously. working now !

Actually, it is normal for the Minimum font size to be set to None because it's difficult to predict what value is safe and will avoid distorting web pages. I don't know what happened to your new tab page in this case!

the small font is also effecting other browser elements , like editing bookmarks , also not legible.

The font size setting on the Settings page usually doesn't affect browser dialogs. There may be something else going on. Are tabs and menu text affected?

Note that changing the minimum font setting from zero can cause problems on website that use 'em' units to set padding and margin, so best is always to leave this at the default 'none' setting.