That's unusual. Which version of Fireworks and what operating system are you using?
If you click on the Faux Italic button, is the italic styling removed? (It should be. If not, click twice.) You might want to try quitting and restarting the application—or shutting down and restarting your computer—and see if this takes care of the problem.
FYI: It's generally a better choice to add bolding via the Font Style dropdown menu, if that style is available within the currently selected font family.