If you just want to test XPaths, you really don't need Selenium IDE.
In Firebug there are two ways to test XPaths. One is the integrated
$x command, which returns different types of results depending on the XPath you are testing. And for advanced testing you can use the extension
FirePath, which selects matching elements within an HTML view and inside the page. Results, which don't return elements are shown within its panel. And if there are no matches, there's a hint at the bottom telling you so.
So if you think one of those two functionalities is "very unreliable", please let us (in case of the
$x command) or the FirePath authors know what's wrong and provide a little test case, so your problem can be reproduced.
Sebastian