While browsing some other blogs, I found this post talking about how FireFox handles whitespace as a DOM node.  The example he gave triggered something in my head, and I realized that we had the same issue in OnTime.

In the web version, you can choose to have datagrid rows highlight when you move over them.  When we implemented multi-edit functionality, we had to change some things that affect the way the row's CSS is changed (related to alternating row colors and such).  The javascript I came up with worked in IE, but for the life of me I couldn't get it to work in Firefox.  We eventually decided to leave it in as is, and the bug got pushed to the back of our minds.  So when I saw the above-mentioned post, it sprang right back to the front.  After some testing, I came up with a way to replace this:

elem = thisRow.childNodes[0].childNodes[0];

with a custom function that would return the first child node of an element that contained a tagName property.  The end result is that in FireFox (as of 5.3, which is still in development), when you mouse over a datagrid row, it will light up, just like its IE bretheren!