The latest version of my File Logging plugin for Selenium IDE has been released. It is now compatible with Firefox 5 and the Selenium IDE v1.1. The log file can also be configured to log the time stamp of each log message. You can turn this on and off from the File logging pane in the options dialog. It also includes some refactoring to bring improvements from my latest plugins. You will get it automatically when AMO approves it. If you want it right now, you can get it directly from the version page on AMO.
This version lacks support for Unicode characters in the log file. Unicode character support is planned for the next release.