Change your user-agent in Firefox

Typically, the user-agent sent by your web browser it way too much verbose. While the user-agent was useful to restrict website features for some browsers that were not supporting recent features, this becomes a concern since a too precise user-agent can :

  • Make it easy for web sites to track you, even without cookies if you have a not so common user-agent.
  • Impair security, especially if you have to browse internet with an outdated web browser.

User-agents examples:

  • Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:92.0) Gecko/20100101 Firefox/92.0
  • Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0

While the first one is a bit too precise and uncommon, the second one is really an invitation for hackers.

How to check it (2 options)

How to change it (2 options)

  • Install an extension such as the user-agent switcher
  • Add the general.useragent.override key/string value in the about:config (recommended)
    You can then set your user agent and test immediately, no restart needed. Feel free to choose a shorter, fake or even empty user-agent. Few examples:
    • Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:99.9)
    • Mozilla/5.0 (X11; Kubuntu) Gecko
    • Mozilla/5.0 (X11) Gecko
    • <empty string>

The first example is obviously fake, since revision 99.9 does not exist yet, and never might.
The other are just fine, with fewer details.

Changing a very different user-agent could cause problems for some poorly written websites. If you feel you’re concerned, at least keep something like the 3rd proposal.

If you need ideas of user-agents:

https://www.whatismybrowser.com/guides/the-latest-user-agent/

Comments

  • The user-agent won’t be synchronized with Firefox Sync. So you have to change it on each device.
  • Some websites stupidly do a quite strict checking on the content-type. This can be the case for some web banking application, or Citrix gateways. In this case, use the user-agent switcher extension, usually those extensions can be configured to present a specific user-agent depending on the visited website.

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