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.           Installing and Applying Themes After you've found a theme you like at Mozilla Update or elsewhere (see "Going to the Theme Park" at the beginning of this chapter), installing it is quick and easy. Follow these steps: Click the Install link. Firefox displays a confirmation window, as shown in Figure 17-4. Figure 17-4: Firefox asks you to confirm theme installation. Click Install to continue with the installation. The Theme Manager opens so you can track the installation progress, as shown in Figure 17-5. Figure 17-5: You can apply, update, preview, or uninstall themes from the Theme Manager. When the installation finishes, select the new theme in the Theme Manager list and click the Use Theme button at the bottom of the window. Restart Firefox if you want to apply the new theme immediately. If not, you can now close the Theme Manager. You must restart Firefox before the theme change takes effect, although you don't have to do it now. To return to the Theme Manager later on, choose Tools Themes.           Updating Themes Firefox automatically checks for updates to (new versions of) your installed themes about once a day and installs any that are available. If you don't want Firefox to check or install updates automatically, you can disable this functionality, as I discuss in the section on updating Firefox in Chapter 15. You can check for updates manually at any time by clicking the Find Updates button at the bottom of the Theme Manager (choose Tools Themes). Some theme authors update their themes to improve the look and feel of the design, but most of the time, designers update their themes to be compatible with a new version of Firefox, which is necessary for the reasons I discuss in "Ensuring Theme Compatibility," earlier in this chapter. When a new version of Firefox is made available, most theme authors update their themes to be compatible with the new version, although it might take them two or more weeks. Although Firefox checks for updates to itself each day at the same time it checks for theme updates, Firefox asks you before installing an update that is incompatible with any of your installed themes. If you proceed with the update, Firefox disables the incompatible themes but continues to check for