I haven't tried
Foxit PDF Reader, but I have heard good things about it.
I would give it a try.
I'll probably install it myself, in the near future, as I'm getting tired of Adobe.
It's sure, a lot smaller (1.66MB) download, than Adobe is.

I also found this extension for Firefox, that you might want to check out.
Looks like it would be a nice compliment to Foxit pdf Reader.
It's a 123 KB download.
PDF DownloadQUOTE
Allows to choose if you want to view a PDF file inside the browser (as PDF or HTML), if you want to view it outside Firefox with your default or custom PDF reader, or if you want to download it!
PDF Download solves the problems everyone has about handling PDF files with Firefox. This extension, every time you click on a link, checks if the target is a pdf file and in this case let you choose what you want to do (open pdf file inside or outside Firefox, download it to the filesystem or view it as HTML).
PDF Download is not a PDF plugin: you still need to have installed the Adobe or the FoxIt PDF plugin if you want to open a PDF inside Firefox.