Truncating Text Rated 1 out of 5 stars
Using the extension Popup Alt Extension truncates the text when it long and overruns the image.
This is the same way the Title Tag works, but is not very helpful if you have multiple smaller images next to each other.
The extension FeatureFix works like expected when converting alt tags in Firefox so they work like IE.
The behavior is not default and you have to set it, but it works and long text is fully displayed.