SQL 2008 MERGE statement was introduced in FB 2.1
Yet when googling about cross-tables UPDATE statement, it is not shown in results.

Old 2003-2005 articles pops up instead, such as
http://www.firebirdsql.org/pdfmanual/MSSQL-to-Firebird-Russian.pdf
http://www.firebirdsql.org/manual/ru/migration-mssql-syntax-ru.html

The latter link, when opened directly from Google does not carry versions disclaimer and does not suggest developer to search for new versions features.

I think the paragraphs mentioning MS SQL extended UPDATE table1 FROM table2 statement should be extended with mentioning MERGE statement in FB 2.1
http://firebirdsql.su/doku.php?id=merge
http://www.mssqltips.com/sqlservertip/1704/using-merge-in-sql-server-to-insert-update-and-delete-at-the-same-time/
http://en.wikipedia.org/wiki/Merge_(SQL)

The whole guide rewriting seems low priority task, that probably would never be done completely.
But this particular problem is too easy to find in Google and i think can be fixed in advance to other obsolete things in those article instances.