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, [...]