Last week Selenium IDE v1.6.0 was released and is available for download from the usual download site. Other than the Firefox 10 support, it has the following enhancements and fixes:-
Keyboard shortcuts to launch Selenium IDE
You can use Ctrl+Alt+S to open the main Selenium IDE or use Ctrl+Shift+S to open Selenium IDE in the sidebar. I guess the equivalent Mac shortcuts should also work. While not new but good to know is that you can customise the Firefox toolbar and add the Selenium IDE button to it without having to install any other addons.
Added break command to autocomplete list
The break command is now autocompleted when you type in the command text box. You can also place a breakpoint on any command by right clicking on the command in the commands table and choosing Toggle breakpoint. These breakpoints will not be saved. So, no need to use the break command unless you want to persistently break at a position every single time you run the test case. Also take a look at my Power Debugger plugin for Selenium IDE. It adds the functionality of breaking automatically when there is an error or a failure of an assert or a verify command.
Improved XPath locator recording when there are multiple matches
When you are recording in Selenium IDE, the XPath locators generated will now try to use numeric predicates for context position filtering when the XPath matches multiple elements. As a very simplified example, if you have a web page containing many images and you click on the second image, the XPath generated could be //img. The  is a numeric predicate that specifies the second element in the matched list should be used, in this case, the second image. Note: these generated locators will not work as expected in IE5.
Locator Builders can now be reordered on Mac
In Selenium IDE v1.3.0, the management of Locator Builders was introduced. Not having a Mac to test on means that bugs like this one creep in. It is now been fixed and you can manage the Locator Builders even on MacOS.
Incorrect tooltip displayed in sidebar
An observant Selenium IDE user noticed that when Selenium IDE is opened in the sidebar, the play test suite button tooltip insisted incorrectly that it was the play current test case button. This identity crisis has now been resolved probably using a heavy dose of sedatives and the power of suggestion (actually it was Leo who fixed it).
If you want to read the original release notes they are at the usual place.