As you can see in the old Developers Guide: http://devnet.kentico.com/docs/5_5/devguide/index.html?system_requirements.htm
IE9 is not officially supported for content editing in v5.5. The reason is that the IE9 was released after Kentico v5.5.
On the other side, I would still recommend you to apply at least the latest hotfix (shouldn't affect your customization) and eventually upgrade to v6. Please understand that you are in the specific state and we had already fixed many issues in the hotfixes, especially about bugs, security updates and improved functionality.