How do I get/install/uninstall/switch/update themes in Firefox?

Getting and Installing Themes
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You can download and install many themes from Mozilla Update.
Internet searches will find many other themes as well.

Usually when you click on a link to install a theme, Firefox
displays a dialog asking you for permission to install the theme. You can
choose to allow the download and installation or to cancel the process.

Uninstalling Themes
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Should you ever wish to remove a theme from Firefox, select Tools > Themes to open the Theme Manager.

In the Theme Manager, select the theme you wish to remove, and click the
Uninstall button. You may need to close and then reopen
Firefox for all changes to take effect.

Switching Themes
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To switch between your installed themes, select Tools
> Themes to open the Theme Manager.

In the Theme Manager, select the theme of your choice, and click
the Use Theme button. It is recommended that you close and then
reopen Firefox so that the theme can be applied correctly.

Updating Themes
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Whenever you would like to check for a newer version of a theme that you
have installed, select Tools > Themes to open
the Theme Manager.

In the Theme Manager, Firefox displays Update Now buttons
for themes with available updates. Click the respective button for the theme
you would like to update.

By default, Firefox will periodically check and notify you when a
new version for one of your installed themes is available. You can change
this behavior in the Update
tab of the Advanced panel of OptionsPreferences.
You can check for updates manually by clicking the Find Updates
button.