Leading 0 on a numerical literal indicates that an octal integer is the intention, unless the second digit is 8 or 9. In that case, the leading 0 is ignored.
Because octal numeric literals must be integers, 02.5 is erroneous.
This behavior was logged as a bug in Firefox in 2014, but because there’s so much JavaScript code in the world and so much of it (probably inadvertently) relies on 09.3 not being a syntax error, the bug was marked “WONTFIX”.
As pointed out in a comment below, in “strict” mode octal constants are disallowed entirely.