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.
- 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)
- Type about:support in your address bar
- Open the Firefox menu → Help → (More) Troubleshooting Information
- https://www.whatismybrowser.com/detect/what-is-my-user-agent (easiest)
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:
- 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.