Memory Leaks

Fighting memory leaks in fancy AJAXy apps is a way of life for most UI developers. Besides Internet Explorer-specific issues, leaks can also occur due to inadvertent remaining references to objects you don't actually need (frequent culprits are global event registries and history/state management stacks). Safari/WebKit turn out to have some useful built-in hooks for tracking these down. By enabling Safari's debug menu and choosing the "Show Caches Window" command from there, you can see JavaScript object counts, force a garbage collection and see the types of objects that still have references pointing to them (note that garbage collection has been improved in WebKit nightly builds so you'll probably want to use that).

Drip can provide some similar information for IE, and Leak Monitor finds some Firefox-specific leak types, but it would be good for browser/add-on creators to provide even more debugging information to authors.

Browser Bugs

WebKit and Firefox/Mozilla both have publicly visible bug tracking systems. As they work on their respective new versions, regressions will occur that will break web sites, possibly including yours. Often, this is a genuine bug in the browser code, and hopefully it'll be fixed before the final release. However, it can sometimes be due to incorrect assumptions made by your code which no longer hold true when browser code is tightened up. Both WebKit's and Mozilla's Bugzilla instances support generating RSS feeds from search results. By subscribing to feeds for bugs that mention your product name, you can stay on top of such bug reports. For example, here's a shared page (with feed) for the bugs that mention Reader on either site.