The memory leaks were serious issues: I've seen v1.5.0 Firefox take up to 123MB when doing heavy duty tabbed browsing and some add-ons: now it uses less then 50MB with the same configuration. Mozilla also fixed a potentially dangerous security hole dating to December 8th. 2005... and people vilify Microsoft for patching the WMF vulnerability in a mere 9 days, but I digress...
They did fix a problem with the latest Java runtime (v1.6 Update 6) which was not always called properly by the browser; this led to several sites not displaying content correctly. My wife and kids are happy because a lot of Flash and Java-based online games now work again.
I noticed a 20% improvement on page loading times after going to 1.5.1: this on a Via-VPSD MII 12000 Mini-ITX machine (not the fastest...hell, a PIII 800Mhz will own this thing in nearly all respects, but since it only uses like 17 watts when browsing, and 23 watts playing a DVD, I'm not going to complain.) I haven't yet tried it on my SMP systems.