I just found out:

select 1 from rdb$database where ('3.4' similar to '[0-9]+.[0-9]+')

works, so the dot seems not have to be escaped, but what is the difference then between [0-9] and [:digit:]?

According to: https://wiki.firebirdsql.org/wiki/index.php?page=SIMILAR+TO, DIGIT means All characters that are numeric digits (0-9)

So I expected this to be the same.