valgrind --dsymutil=yes ./firefox-bin http://www.mashable.com will attach.
Charles, this looks like something you guys probably should have a look at. You'll probably want to repeat this in a build with symbols etc to get more useful data out of valgrind. Let us know if there's anything we can do to help!
Picking this up.
Resolving old bugs which are likely not relevant any more, since NPAPI plugins are deprecated.