Fonts are smaller in Firefox than under Gnome.
If you want to fix that small (and hopefully temporary) issue, under Firefox type in the URL bar :
Then in the filter box, search for “dpi”
This will filter out the value “layout.css.dpi” having a default value of “-1”
Change that value to “0” and restart Firefox, the fonts will be the same size as under Gnome.
Edit : Apparently, it is fixed in apha 4.
Edit 2 : issue back in alpha 5 (upgraded from alpha 4)
Edit 3 : Tim is reporting the issue is not present on a fresh alpha 5 install…