I bought my MacBook in 2008 and it cannot be upgraded.  Apple only support their current OS and the one before.  I'm about 4 behind now.  Effectively, I am stumped.  I can't even transfer my itunes tracks onwards, cos my itunes format (which cannot be upgraded) is too far behind to be compatible with any hardware I buy now.

So, for me, my perfectly working 5-yr old Macbook is effectively obsolete for anything involving connecting to the internet.  Firefox kept their support going longest, but even they bowed out at the beginning of this yr.