I'm another one that has found 1.5 more stable than 1.0, I would often get cases where Firefox and Thunderbird would hang at the same time with 1.0, which doesn't happen with the latest version.

The only outstanding problem that I have with Firefox is that after a specific instance of it has been running a couple of days it will suddenly decide that its window is much wider than it actually is, resulting in some of the right hand side of pages (and the address bar) being clipped. Closing and reopening fixes this.

While it appears to use more memory than IE, it is by no means a memory hog. Even when I have had an instance running for a few days, if I close all the tabs it is only using about 120MB.

This is on Windows, when I have used Firefox on the Mac it didn't seem to be quite as stable.