Each theme is designed to work with a particular version of Firefox or a range of versions. This seems strange at first, but makes more sense when you consider that each version of Firefox adds some new features. A designer has trouble theming a feature that hasn't been invented yet, just as a painter can't paint a room that hasn't been built.
Firefox prevents you from installing an incompatible theme, but you can save yourself some trouble by ensuring a theme's compatibility before installing it. Mozilla Update provides a theme's compatibility information below its Install link in the form of a range of Firefox versions that it supports (for example, Firefox 1.0-1.5). If a theme supports only one version, the endpoints of the range are identical (for example, Firefox 1.0-1.0). Make sure that your version of Firefox, which you can find out by choosing Help About Mozilla Firefox), falls within the listed range.
Even if you use a theme that's compatible with your version of Firefox, problems can occur if you're using an extension that changes or extends the Firefox interface. (See Chapter 20 for information about extensions.) Firefox does the best it can to apply your theme to installed extensions, but some things might look funny or stop working. In such cases, you will need to switch back to the default Firefox theme or disable the problematic extension.
Whenever you update Firefox to a new version, it checks to make sure that your installed themes remain compatible. If it finds any that aren't, Firefox notifies you and disables the incompatible themes until their designers release updated versions. Firefox checks for updates about once a day, but you can check manually at any time, as I explain in "Updating Themes," later in this chapter.