Toolbar icons overlap in Firefox

When using firefox some icons (not all) are overlapping each other, in IE it works ok. I work with one XML file and switch icons on or off as needed.

Any suggestions where to look or is this a known problem?

Please check that in your XML file size of buttons set, such problem can occur if size of button not set or set incorrectly.
If problem still occurs for you - please provide any kind of sample where problem can be reconstructed ( you can send it directly to )