For the pure basics please visit the MacroLanguage documentation in the K-Meleon Wiki.
Besides true tabs, K-Meleon 1.5 got a new macro parser. This one is more strict and will notify about errors that were not discovered in previous versions (especially in regard to missing semicolons and malformed strings). Macros have to be encoded in UTF-8 now.
The syntax of the injectJS() and while() statements was extended. New methods opentab() and openbgtab(), new events OnOpenTab and OnCloseTab, and a new global variable $TabNumber were added to allow working with tabs.
More (documentation) to come...