Caution.I take your point re support, but would add a couple of things

If your macbook is circa 2008 it should be core 2 duo, not core duo,( which was 32 bit based pentium m), and capable of running Lion assuming at least 2 gig memory and 4 gig HD space
Most from late 2006 onwards most were core2, and you can check in system profiler to confirm

Mountain Lion would not install directly without 3rd part support, but should with
Main problem was Apple dropped 32 bit support with ML and also didn't update older graphics chipset drivers
but if manufactured circa 2008, yours will be 64 bit and likely have nvida graphics chipset, which should make things easier re installing ML with 3rd party support

To be fair, browser support for older OS' is a matter for the software supplier, and for similar reasons to macs being subject to less malware ie namely the small numbers involved, they may not consider it worth spending resources supporting older OS. Thats largely a numbers game. As for vulnerabilities, you have an existing edge because of the relatively small numbers involved in terms of users, and to an extent software vendors have a tendency to over stress such things

You could ironically install windows 8, which I understand does support the older chipsets in question
It may be worth checking the spec in profiler against apples requirements for Lion