AVG Security Toolbar installs itself to Firefox.
If the AVG Security Toolbar is installed to the local Firefox installation, it will attach itself to instances of Firefox Portable included in the Tor IM Browser Bundle 1.3.10 and previous.
It's major because:
AVG Anti-Virus Free updates itself without user intervention. If it was installed with the "AVG Security Toolbar" option checked, and Firefox was installed after AVG, the toolbar will not be in Firefox. However, when AVG auto-updates, it will install the toolbar as a Firefox add-on without the user anything.
There is evidence that the toolbar communicates user nationality to power the search field. Correctly localized web pages and news articles are served back to the user even through Tor. The search engine knows the user's language and location settings as they appear to AVG.
It automatically pastes searches done in the Firefox search bar into the AVG Security Toolbar search bar. I don't know to what degree that information is sent to their or search engine servers.
The computer's administrator may have set the default search engine to Baidu, which is located in China.
The user may be unable to change anti-virus settings.
To easily replicate this issue, install AVG Anti-Virus Free (no charge) to a Windows machine with Firefox and select the option to display the AVG Security Toolbar. Or, install Firefox to a Windows machine with AVG Anti-Virus Free, download the latest AVG Anti-Virus Free, Launch it and select "Repair Installation." If AVG auto-updates, it will do this step for you without any prompts except to reboot when it's done! Then launch a Tor Browser instance and a functioning AVG toolbar will be there.
To remedy the issue, an administrator can "Repair Installation" and uncheck the AVG toolbar option, or disable the add-on in Firefox.