User Agent Switcher Help


The User Agent Switcher extension is available for Firefox and will run on any platform that this browser supports including Windows, OS X and Linux.


If you are having problems installing the extension then please try the following:

  • If you are having problems upgrading the extension then please uninstall the extension before upgrading. If the browser is unusable after upgrading then try starting the browser in safe mode and uninstalling the extension.
  • Try restarting your computer and installing the extension again.
  • Try saving the extension to your machine (by right-clicking or ctrl-clicking the download link and choosing 'Save Link As...') and dragging the saved extension into your browser window to start the install.
  • Try installing the extension in a new profile.


The extension can be uninstalled using the built-in uninstall functionality provided by Firefox.


Why does the extension not work in Firefox 57+?

Firefox 57 introduced a new extension framework and all extensions need to be updated to work in Firefox 57 or later.

Why does the extension not work on some sites?

Switching the user agent is not guaranteed to fix issues with every site as sites can use a number of methods to detect the browser some of which the extension does not support. Sites can also use non-standard code that does not work in the browser and switching the user agent does not make the browser render a page like another browser.

If you find a site that still does not work after switching the user agent you should contact the webmaster of the site and ask them to support your browser.

Why does the user agent reset when the browser closes?

Due to a bug in Sun's Java plug-in the extension automatically resets the user agent when the browser closes in order to stop the browser crashing on the next start-up.

If you are unable to start the browser find the file 'prefs.js' in your profile and delete the line containing 'general.useragent.override' to fix the issue.

If you understand this issue and want to disable the auto-reset functionality create a new preference useragentswitcher.reset.onclose and set it to false. No support is provided for this configuration.

How do I add the Googlebot or iPhone user agent to the list of user agents?

The Googlebot and iPhone user agents were added to the extension in version 0.7. If you do not see these user agents in the extension you should reset the extension options by selecting 'Edit User Agents...' under the 'User Agent Switcher' menu and clicking 'Reset Options...'.

Will you add a new user agent to the defaults?

No. The extension allows the user to configure the list of user agents. The defaults are provided purely as examples. This is an extensive list of user agents that can be imported in the extension options and others can be found with a search on Google.

Why are there no default user agents when the extension is installed?

This is a bug. You can work around this by saving this default list of user agents as an XML file and importing it in the extension options. This will be fixed in the next release.

Why can the vendor and vendorSub properties not be overidden in Firefox 4 and later?

Mozilla removed the ability to override the vendor and vendorSub properties in Firefox 4.

Why does feature 'X' not work in a nightly, alpha, beta or release candidate browser build?

Nightly, alpha, beta and release candidate browser builds are not supported. If you see the same problem with a milestone build then please send me an email.

What should I do if my question is not covered here?

Please send me an email.