When upgrading to Fireboard 1.0.4 several folks have reported legacy html tags to show up in their posts and signatures. This is a relict of the old - less stringent bbCode parser as well as migrations from other non Fireboard forums, that allow html tags inside posts and signatures. The new parser is much more strict about html with its natural XSS attack prevention.
In order for older posts to get displayed correctly we have put together a patch script (sql) that you can execute against your FireBoard database from within phpadmin. It will convert these tags to proper Fireboard bbCode syntax. We highly recommend you make a backup of your database before executing it. However, it can be run repeatedly if necessary.

I have added a little section to the Known Issues release notes for 1.0.4 on the wiki.

You can also get the patch directly here.

Good luck!

Many thanks to numenus for providing extra sample data!