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 updates to them each day. When an update is available, Firefox installs it automatically and allows you to switch back to the now-compatible theme.
Each theme has a version number. The designer uses this number to keep track of the themes. You don't need to worry about it; just remember that the higher the number, the newer the theme.