Need an in-browser PDF reader? Sure you could go with Adobe Reader, but who wants that bloated mess? If you want something quick, built in and simple you need to go with the Nodobe add-on for Firefox.
At this time Nodobe support 38 different file format:
Adobe PDF:
pdf
Microsoft Office:
doc, docx, rtf, xls, xlsx, ppt, pptx
Images:
png, jpg, gif, tif, bmp, ppm, xpm
Vector Graphics:
eps, ras
Text Files:
txt
OpenOffice:
odt, odf, ott, odg, odp, stw, sxw, std, sxd, sti, sxi, sxc
StarOffice:
sda, sdd, sdw, vor
OpenDocument:
otg, stp, ods, pts
You can pick up the Nodobe Viewer on its official web site or the Firefox Add-ons web site. Free yourself from Adobe, and give it a shot today. Also if you know of anymore Adobe Reader alternatives, be sure to share!

Time to grab those open source grapefruits and squeeze them till we get all the goodness out. Been heading here and there finding the best Firefox news one guy could possibly link to. What you see is grade A quality stuff. 