I have two PCs I use on a regular basis; my old HP Compax Nx9110, and my new Dell XPS system. Both have the same updated version of Firefox running on them. But the Dell renders webpages differently than the HP does; in particular, layouts tend to get screwed up all the time (overlapping columns, boxes, etc.). I've noticed this both on LiveJournal and Woot.com.
Any possible idea why this is going on? It's downright annoying.