SeaMonkey is the latest incarnation of the Mozilla Suite, which grew out of Netscape Communicator. Essentially, Firefox uses the same rendering engine (Gecko) (and I say that Firefox uses the same and not that SeaMonkey uses the same as Firefox, because frankly, SeaMonkey has been around a lot longer than Firefox).
I also run Firefox, but 99% of my browsing is under SeaMonkey. ;-)
Looking at http://www.blue-mount.com/portfolio/shanghai-tangs-sale-signs/ , I can see that our first hurdle would be dealing with the slideshow (i.e., which image do we want in the PDF?). This is probably where another suggestion regarding image thumbnails might come into play.
Looking at the source for http://www.blue-mount.com/sale-time-again/ , the images themselves seem to be in the expected directory path. However, I also see references in the css to printfriendly; do you happen to have the PrintFriendly plugin installed, and is/was it active when you tried WP Post to PDF?
You might want to try toggling the "Other Plugins" presentation option in the WP Post to PDF settings, too. This might get around another plugin which is changing the image urls before we're getting them.
PS - Those two pages seem to be have better for me than the About page, too. At least I'm not seeing my RAM & CPU climb out of control.