Thanks for the report. This is a known bug in Firefox's code to enlarge text to make it more readable on small screens. For details, see https://bugzilla.mozilla.org/show_bug.cgi?id=707195

As a work-around until this is fixed, you can disable text enlargement by changing the "Text size" setting to "Tiny" in the Firefox settings screen.

I started experiencing tiny tiny fonts on two forums I frequent, Ravelry and a Yuku board. Can't find a way to fix it. I think it started with the most recent Firefox update. HELP!

As mbrubeck explains. Font inflation can be disabled by opening the Firefox settings by either pressing the Android hardware menu button or the 3 dot button in the upper right of Firefox. In the Firefox Settings select text size and press the small a several times till the text does not get smaller. Then press Set.