Mozilla has advised Firefox users not to try add-ons classed as 'experimental', such as the recently released China Channel, because they could cause computer problems.

Firefox add-ons allow users to personalise their browsing and add additional features. Because Firefox is open source, many of these add-ons are created by third-party developers.

Add-ons classed as 'experimental' are still being tested, Mozilla warned, although they are available for public use.

The company does not recommend downloading such add-ons unless the user is very technically minded. "They have not been approved, so they may pose risks to users," said a spokeswoman.

The China Channel experiment received international press coverage on its launch because it allows users to surf the web as if they were in China.

The coverage came at a time when internet censoring in certain countries was attracting the attention of human rights groups and multinationals.

This, in turn, prompted the launch of the Global Network Initiative, a statement by organisations including Google, Yahoo and Microsoft, to stand up against government policies that curb freedom of expression on the internet.

More than 1,000 Firefox users downloaded the China Channel, which does not appear at first glance to be problematic.

"It's open source, free and easy," reads the add-on landing page. "Take an unforgettable virtual trip to China and experience the technical expertise of the Chinese Ministry of Information Industry."

However, a test conducted by vnunet.com found that the pages to which the China Channel is supposed to restrict access seem to be lost for good, even when the add-on is disabled.

Other testers appear to have had problems as well. One user who commented on the China Channel site said that he had experienced an "uninstall bug", and that all proxy settings were lost after he closed Firefox.

Mozilla pointed users to the small print attached to add-ons. When users are given a choice to install an add-on, they can click on a 'why?' button which offers the following information:

'This add-on is meant for advanced users to test add-ons before they are made available and reviewed for general use. Many add-ons may be in prototype form. Experimental add-ons may be alpha, beta or pre-production in quality, performance and features. Caution should be used when installing experimental add-ons, as they have not been tested by an editor and may harm your computer configuration.'

The Firefox UK marketing team said that it was unaware that the add-on had received widespread press coverage, and plans to investigate how a potentially dangerous application is quickly becoming a popular experiment.

In related news, Mozilla yesterday launched Firefox China Edition.