Hello Lukáš,
When the JBos Source Lookup searches source for some class, the class's FQN is only available. It can't know if the class is from JSF 1.2 or 2.1.
The Source Lookup plugin uses the first library in which it finds the class to search the source. If there are more versions of the same class in the JBoss AS distribution, you won't get the correct source in all the cases.
The best way to solve this issue would be to download the source of the JSF implementation you are using and add it manually to the Source tab of the Server Launch configuration.
If that source library is placed before JBoss AS container, Eclipse will show that class.
The Source tab is got by double-clicking the server in the Servers view and clicking the Open launch configuration button.
Thank you for your feedback.
BTW
The Source Lookup plugin will be included in the JBoss Tools 3.3.0.Beta2 - See https://issues.jboss.org/browse/JBIDE-9309?focusedCommentId=12688252&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12688252