First PDF I loaded was a fairly simply W2 (US tax document). 1 Page, standard text inside of boxes, and Firefox pegged a core the entire time it was open. Yet another point in Chrome's favor, which rendered the document perfectly.
Can you share the link so we can see the problem too?
I think Mozilla mainly intends this viewer to be a previewer, to handle the 99% of cases where someone just wants to quickly browse through a PDF and then forget it. If you want advanced functionality, forms, etc. then i think they would still recommend sticking with Adobe's PDF viewer, or whatever plugin you are currently using.
No, I actually won't share the link to my W-2. It's a rather personal financial document. It is not a "form" in that it has data input, however.
Here's a sample of a different doc, it doesn't peg my CPU, but Firefox' memory usage shoots up about 300 megabytes to render and scroll the form.
http://www.irs.gov/pub/irs-pdf/fw2.pdf
If you are using Linux then Evince is great.
If you are using Windows then Sumatra PDF is great.