Incredibly enough, it seems to be caused by the anti-popunder surrogate. I still can't figure out why, though (it has nothing to do with fonts and is not causing any error on those pages, so it must be a subtler Gecko < 1.9.1 bug).
You can work around by setting the
noscript.surrogate.popunder.sources about:config preference to an empty string for now, or by adding the malfunctioning domains to
noscript.surrogate.popunder.exceptions.
[EDIT]:
it turns out not being even popunder-specific: any page-level surrogate execution causes this font increasing on those pages. The mystery deepens...
Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 (.NET CLR 3.5.30729)