I installed the plugin from the last of Kris' links - and I swear I did a search of Firefox add-ons at the mozilla.org site and that one (which is the same plugin as Kris linked to) did NOT show up. Weird. (Shows up now) Also, there is no documentation that indicates that it DOES work on Intel Macs which isn't very helpful, since Mozilla states so categorically in several locations that there are no plug-ins for viewing PDFs inline on Intel Macs. I suppose the operative word here is "supported" - and they don't support plug-ins listed as "experimental"? You'd need to read the comments to know that - and even then, there is only one comment at about #25 that mentions Intel Macs. Google turns up a lot of out-of-date results, though there was one near the bottom of the first page of results that linked to this plugin (now that I know what I'm actually looking for!!).
Somebody needs to do a better job of updating Mozilla's info - it's all well and good for them to say that there is no "supported" plug-in, but it might be nice if they'd mention that there IS an experimental one for those who don't mind living on the bleeding edge. And given the comments, it's also not much of a bleeding edge at this point...

BTW - if you install from the Google link, there is no requirement to log in to install the "experimental" plugin, like there is from the Firefox Add-On page.